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.