Also, what about their families? Not all of them are single white males in their 20s. Their partners would probably want to keep working, not necessarily on qualified jobs that could grant a visa anywhere in the world.
What I'm saying is that is not as easy as it might seem. People have different situations to account for.
Besides, having passed that exam myself I actually think it was a good experience. I learned a few things about the UK.
Language I can see, but then the German Abitur has a requirement of 2 foreign languages, with potentially 3 (English/French, French/Latin, last one depending on what the school offers, usually Spanish/Italian/I think Russian in NE/sometimes Chinese). Some of these languages are substantially similar, which further widens the field, e.g., a French speaker might transition to Italian easily.
A fair number of Europeans are ridiculously multilingual.
Either way, they'd need to learn more if they wanted to have a normal life in an English-speaking country.
https://www.gov.uk/apply-for-a-uk-residence-card/permanent-r...
The US has a very restrictive visa system. It didn't prevent the development of the silicon valley.