why do long term expats/immigrants stop improving?

I've met several people that live in a foreign country for a long time and speak the language in their daily lives but somehow their language ability doesn't seem to improve after a certain point. like when people first move to a new country they seem to get better for the first few years but even those who've been in a country for ten years many of them say their language ability is more or less the same as 5 years ago. why is this?