In more concrete terms: 60-70k EUR is what you are looking at in Germany in good job markets, while in Poland the figure is 30-40k EUR. When you consider taxes, housing, and other costs, the difference is really not that great.
The big difference is in the savings. Even if you just make twice as much, your savings per month can be several times higher than in Poland, which makes a lot of difference if you want to go back to your country later on and buy a house etc.
Also, yes, a simple life means not to live like an American expat lol. You can have a social life without spending lots of money if you socialize with "like minded" people who also don't want to waste money. You can have friends over for dinner at home with a few beers from the supermarket, play cards or boardgames or video games, soccer or basketball outside, have a BBQ, watch a movie at home, etc.
But it's not like you'd have an American expat-like luxury life in the alternative scenario of staying in Eastern Europe either. The point is, by keeping down your lifestyle you can multiply your savings and escape the cycle of poverty and be able to afford a home in a few years, if you go back to your home country and can support your relatives back home in the meantime. This is a great deal for a lot of people even without the fancy expat-like money-burning lifestyle.
University towns only really make a difference if the hypothetical person moving there is college age and if there's a significant number of foreign students. Otherwise why would German college kids want to socialize with a late 20s foreigner?
Someone which moves to a new country will leave their entire social circle behind: family, friends, personal and professional acquaintances. It's an immense loss that will never really be recovered and the older you are the harder it is.
Now lets cut to the chase. With the exception of the top 40 cities in Germany every other city has a population of less than 200k people. Unless you go to those 40 places which represent roughly 25% of the population the rest of Germany is nothing but "small cities".
I have some friends living in what according to wikipedia one could call a half-town and looked at an apartment there out of curiosity. A 3 room apartment (newly built) was about 280.000 EUR and that's in a town where they have 1 hotel, 1 restaurant (or maybe 2) and 0 supermarkets. In my experience if one wants a 3 room apartment in a normal city, one would add another 100.000 EUR to that price.
I've made the opposite experience. Sure everything is 30% more expensive in Germany than in Poland but you also make twice as much money. The differences in cost of living are pretty insignificant but the salary is so much higher that it more than makes up the difference.