How does insulating footwear prevent an electric shock? The reason I have always heard to explain the reduction of electric shock when we wear insulating footwear goes as follows:

When electricity passes from our body to the ground, an electric circuit is complete. This results in flow of charges resulting in an electric shock. However, an insulator makes this circuit open by not allowing the current to pass through.

But when charges enter our body, even though they do not flow and go inside the earth, they are still present in our body, right? If there's an excess of electrons, we should feel the additional electrons' presence regardless of whether those electrons flow through our body or not.
What am I getting wrong?
 A: Take a capacitor and put it across a battery. There will be a transient current as the electrons   go towards the anode . This happens very fast and the current is small. If you short the capacitor with a wire, the battery will empty all its charge on the short, which, depending on the battery can really be damaging.
Your body accumulates some charge which the insulating layer of the shoe does not allow a short to the ground to happen, as with a capacitor. 
If one is working around charges it is wise to have non-conductive shoes in case of accidents. The power charging up the body will be nothing compared to the power if the current found a short to the ground.
p.s. Have you seen birds sitting on high voltage lines? They probably feel a small prick when they charge upbut no current goes through them, unless their wing touches a neutral. Fact: my son when doing his army duty close to a forest  helped put out a fire started by a large crow who when flying off from the high voltage shorted with its wings to the neutral , caught fire and fell on the forest floor starting the fire.
A: The whole electrical power grid is connected to ground.  I don't know the details of other regions, but if you are in North America, the two current carrying conductors in a residential electrical outlet are called "hot" and "neutral".  The "neutral" conductor is connected to the Earth at many places.
If your bare feet touch wet Earth, and your hand touches the "hot" wire, it's almost the same as if your feet were connected directly to the neutral wire.
A: If you are not in a complete electrical circuit, any electric shock caused by touching a charged object or wire is brief. These "static shocks" are slightly painful, but they are (rarely) dangerous or fatal. I'm sure you've experienced a minor static shock.
By wearing insulating footwear, you break a complete circuit and forbid a flow of electricity from passing through your body. These electric shocks are very dangerous.
A: If you have an excess of electron in your body, your hair might stand on end and you might feel a bit negative (I couldn't help that pun), and you should probably avoid touching people or metal object if you don't want a static shock, but other than that, it's mostly harmless. The real danger comes from flowing electrons. Because the body basically runs on electric impulses with very small currents and voltages, even a tiny amount of current flowing through us could do drastic harm. When you get a lot of electrons flowing through you, they could cause all sorts of mayhem. From burning you to stopping your heart.
Normally, in bare feet for instance, when you touch some highly charged object, your body provides a conduit for that charge to flow through you and into the ground, the place that charges tend to want to go. This is bad because it's flowing electrons. But when you wear insulating footwear, you block the charge's ability to go to the ground. Instead, all that happens is the charge between you and the object balances. You gain or lose some electrons. Sure, there's electrons flowing through your finger or your hand, but nothing important like your heart or major nerves. Worst case is you get a slight burn where you got the static shock. Then the charge stays in you until it finds a way to jump to something less charged. Keep a small metal thing in your pocket and use it to touch anything metal when this happens. That prevents you from getting a shock. But ultimately, the electrons aren't flowing through your body. They just sit there. No flowing electrons, no major problem.
A: Typically this is explained by the saying, "current kills."
It's not the charge (or potential above ground) that a body attains that hurts biological systems, it's the current that flows through them and either 1) heats them or 2) disrupts important electrical signals in the body.
Heating damage occurs and can "cook" (cause 1st, 2nd, or 3rd degree burns internally or externally) portions of the body where the current flows.
Electrical disruption is the more deadly of the two, commonly.  The heart depends on regular electrical pulses from the sinoatrial node to contract not just rhythmically, but for the entire heart to contract at the same time. If an electrical current is induced across the heart at the wrong time, or across a weak heart it may lead to  atrial fibrillation.  This is the most common abnormal heartbeat pattern the heart ends up following when presented with such an electrical interruption, and it's extraordinarily inefficient, leading to low blood oxygen.  AED (automatic external defibrillators) are designed to detect this abnormal beat, then apply a specific electrical current and waveform to the heart to give it a good chance of regaining normal beating. With very high voltages, we can discuss the muscle contraction that takes place, and the possibility that this would cause the person some damage depending on their location (ie, throwing themselves backward into a concrete wall due to a sudden jolt), but this wouldn't be the case for the situation at hand.
While heating does occur any time there's any current flow (except in a superconductor, theoretically), it takes a lot of current to cook organic materials.
While the heart can react to electrical currents, it's not very sensitive.
Thus it's the current that flows through the body, or the heart in the case of fibrillation, that is important.
The charge required to bring a human body up to the potential of the circuit they are touching is very, very small.  Effectively the human and ground becomes a capacitor with an insulator between them - a small one on the feet, and the air as insulator elsewhere.  The capacitance would be very small as the insulator is neither thin nor large (the closer the two conducting surfaces, and the larger the surfaces, the greater the capacitance).
So it would only take a very small amount of current to charge up the body, and that current isn't significant enough to cause either heating or electrical signal disruption.
Further, from the point of contact, the charge would rapidly radiate throughout the relatively conductive body, meaning that the area of greatest current is the contact point, but after that no other spot on the body receives any significant amount of current. Even if you were able to increase the capacitance significantly, and you chose a very, very high voltage, thus forcing many more electrons into the body, the only part that would become damaged would be the heating damage at the point of contact with the conductor providing the charge.  You might get a small electrical burn there.
So the situation you describe could only generate a very, very small current to bring the body up to a specific voltage potential, and thus would be unlikely to produce any damage.
That being said, don't do it anyway. Take all reasonable precautions.
