Remember that sound intensity is the amount of energy flowing through an area normal to the surface:
The surface area of a sphere is $A = 4 \pi r^2$. That's why intensity scales as the inverse square of the radius for point sources.
shouldn't the power (p∝A) of a sound/point source wave decrease with distance?
Be careful! This is one of the most common misconceptions for students starting out in acoustics. Power is a property of the source, and so it doesn't change with distance. Think about it this way: power is just the rate at which energy is being changed from one form to another.
If you're familiar with calculus, another way we can see how power doesn't change with distance is by looking at the following integral:
$$P = \oint \vec I \cdot \vec n \ dS$$
It says if we take the intensity over an entire closed surface (perpendicular to the surface) we'll get the power of the source. The energy of the source has simply been "stretched out" over a larger surface area with distance. Such an approach is convenient because as long as you don't have sinks of acoustic energy inside the surface you can quantify the total power of multiple sources inside of the surface - this is nice because most real-life sources (like an engine) are actually aggregates of myriad small sources of sound.
In fact, intensity measurements are a way many engineers quantify the sound power of various machines.