Because their countries have virtually no film industry.
Not necessarily true. There's definitely a ceiling in the British TV and film industry, which makes it way more lucrative to work in Hollywood.
If Idris Elba stays in England, and he's never in The Wire, his high water mark is 'Family Affairs'. If he's lucky he lands 'Luther' and they either let him carry on until he passes away (in which case he's typecast as Luther) or they cancel it one day and everyone forgets about it.
In theory, the American film industry has so few ceilings, you can be a mediocre actor and then literally become president of the country, one day
The British film industry exists, it just places really high value on American acceptance.