Edit: and also the little transparent bit where you see how much water is inside. Hard to make that out of plastic or wood. The electrical cable also contains plastic but that seems important.

A lot of the small windows have them be made of plastic, with or without a little plastic bead in them.

It's the bigger ones where basically the whole kettle is clear that have it be made of glass.

Edit edit: Now that I think about it, a 100% metal electric kettle sounds terrifying.

Either that, or it will stop working very quickly. The only way I could see a kettle like that working is if the kettle itself was completely inert, and it had an induction base that went along with it.