- People drink at all times of day, and
- Household power sockets can deliver a lot of power (a typical $30 kettle in the UK is rated at 3kW vs. US where 1.8kW is normal even for more expensive ones)
Anyway, before kettles became cordless[0], they all had detachable leads. And there was a standard. So you could use any 'kettle lead' with any brand or model of kettle.
To this day I still use 'kettle lead' to refer to the type of cable used to power a desktop PC.
[0] the kettle itself has no cord, but the base has a hard-wired cord