Europe is not a monolith and I don't know why people think it is. 2 years stints are not unusual at all in London.
It's about equity worth nothing jn Europe as an employee. Europe is a bad place for employees to join a startup. Lots of time people are attracted with shares/stars whatever. Only to find out they get nothing or are taxed to hell. There is a reason why Europe is failing.
London is economically, politically, geographically, and culturally fairly distinct from continental Europe.