Op, your original question is asked from a position of privilege - you live in the global North, and I assume that you have not lived through communism, authoritarianism, a failing state, warfare, genocide, hyperinflation, arbitrary detention, removal of minority rights etc.
Well, the entire post-soviet bloc including Poland and East Germany, the Baltics, Croatia, Bosnia, Serbia, Albania etc - have all those things in their very recent political memory (last round of warfare and systematic human rights abuse by a government, on European territory, took place in 1999, for example.
So for a large chunk of Europe, the EU is a democratic project - countries emerging from communism and warfare, for example, were able to democratise because EU membership was the end goal of their transition.
When 'old regimes' ended, countries didn't become democratic and problem free overnight so the idea was that working towards membership would promote reform and good governance at all levels. I am simplifying massively here for brevity, there were and are problems with this system.
However, meeting membership criteria was basically a major goal of pretty much every post communist government at some point, and many, like Poland, implemented laws and democratic.reforms.that they wouldn't have otherwise.
As a result of 'aiming to meet European standards' for membership, many countries also reformed their policing practices, domestic violence support, anti pollution and building regulations, workplace rights, minority rights, basic humane conditions for prisoners, I could go on.
Bottom line, if you are a citizen in a corrupt, post-authoritarian system (and if I remember correctly, citizens in Bulgaria had to get a visa to leave their own country) where your basic rights are severely compromised, the the EU has been a.major force for democratic change.
Hasn't always been perfect, but seriously, if you'd experienced the political turmoil that much of Europe has until recently, then its a no brainer.