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.
Answer
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.
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