For the ~500 million people of the EU, moving to Frankfurt means taking a train there, moving to London is a whole headache of visas, permits and permissions.
Founder visas are generally suffering from a chicken-and-egg problem, where only a successful company can sponsor anyone
Sure, but the entire VC and funding ecosystem that London has is nonexistent in much of the rest of Europe.
It's easier to raise rounds with better terms in London versus mainland Europe, aside from CEE where diaspora VCs in the US tend to step in to build the ecosystem.
But even then the entire ecosystem pales in comparison to the US.