Why are people assuming this is a tax issue? It's just most people find it nicer to live in the EU, even billionaires, and especially if you don't need to worry about work.
Apart from that quite a lot of other EU members offer residency for investors, not ideal either but better than outright sales of passports.
If you aren't rich the rules for citizenship vary quite a bit between member states but they all require some years of residency and tax payments to qualify for citizenship.
Here in Sweden I'm going through the process and it's one of the most straightforward citizenship processes: live here for 5 years legally on a visa, pay your taxes and you qualify.
Here are the first two links on DDG:
https://www.second-citizenship.org/permanent-residence/portu...
https://www.escapeartist.com/blog/get-residency-second-passp...