Poland’s admission to EU was decided first by their citizens. But with the benefits came certain rules. Rules that all countries have to respect.
You can’t “have the cake and eat it too”.
Are you so easily sure that some EU law is above all constitutions?
Absolutely. Even without legal reason, common sense can explain this. Signing treaties with other nations is entirely pointless if anyone can just say: “sorry, but our obligations from the treaty are against our constitution, so we will ignore them.” As a result, international law often has priority over national constitutions. Being a member of the EU is exactly such a type of treaty.
Only those constitutions belonging to countries which join the EU, in the process agreeing that this is the case.