Why don't most of the highly educated people in poorer part of Europe migrate to the Western part? There are a lot of people working in universities in countries like Ukraine, Belarus, Romania, Bulgaria, Poland, and so on with low salaries. Those people have high skills and education.

I saw some students pursuing PhDs in those countries with low salaries when they can just cross the border to earn much more.

Why don't most of them migrate to Western Europe?

What factors bind them in their countries?
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.