Why aren't (domestic) kettles insulated? In my experience of buying and using kettles, I have come across none which are insulated.
The obvious reasons as to why it would be beneficial being that heating time would be reduced, similarly, less power hence money would be required to heat an arbitrary volume of water. Some kettles become very hot on the outside so safety is also a factor!
Is there a reason why this is so, apart from the costs involved? I.e. cost of manufacture vs. operating cost over the product lifetime.
 A: Most kettles are silver to minimize heat loss through radiation. (They also have small exit holes at the top to minimize heat loss of steam because conversion of liquid water to steam requires latent heat)
I expect the reason that there is usually no thermal insulation is that kettles heat water very quickly and because the air outside the kettle is a poor conductor of heat the ammount of heat lost by conduction/convection is probably minimal compared to the ammount of enrgy that goes into the heating of the liquid to make it boil. 
By contrast, a hot water tank in a central heating system stores hot water for long periods of time, so it makes sense to carefully insulate hot water tanks.
In summary, I think one way to think about this is that the air outside the kettle is good enough thermal insulation for the short time that the kettle boils the water. 
A: I disagree with this answer, and am also very frustrated to not find more insulated kettles to replace mine that has unfortunately broken (a Magimix).
An insulated kettle stays warm for much longer, so saves a lot of energy when the kettle is reboiled after a period of say 5 minutes to half an hour, which is a very common scenario, and is a bigger energy factor than the heat loss during boiling (which is also in favour of the insulated kettle, as well as the higher power kettles, which have now been banned in the EU where I am).
As noted in the original question, an insulated kettle is also quieter and safer - our replacement kettle (a Cusinart) kept burning users.
The heat loss outside a kettle is in fact governed by convection of the air, outside the tiny laminar sublayer where conduction is the main effect. Air might be a poor conductr, but it is a great convector - otherwise the old-style hot water fed 'radiators' which are in fact 'convectors', would not work. A few mm of insulation within the kettle wall makes a big difference.
What is amzaing/shocking, especially in this day of energy/ global warming awareness, is that I have yet to see any kettle review mention whther a kettle is insulated, and it is almost as impossible to tell from the specs.
A plastic-walled kettle is a better inulator than a steel one, is associated with a (low) budget quality.
