Why don't most of the highly educated people in poorer part of Europe migrate to the Western part?
There are numerous reasons why one would want to stay in a particular country even though earning power is lower.  These are usually summarized as "frictions" in the labor market.

Culture, familiarity with an area, language, family are all non-negligible "frictions".

I note that while there will be a tendency to vote this closed as not economic, I think the reminder that there are meaningful human incentives other than money and tangible goods may be worth remembering.