Probably the reason is more political and cultural than economic.

FWIW, the origin of "third world" is political, being the NATO and the Warsaw Pact countries the first and second. The poverty implication came later.

Is a visa needed to enter Spain from any Spanish-speaking country? (Spain is in Schengen)

They already have an income requirement. How does this contribute to better enjoying a good quality of life?

Also discussed in this subthread https://news.ycombinator.com/item?id=35344437. My take (having been there two weeks and done all the tours and seen all the things) is that you'd need (less than?) half of that income requirement for a normal comfort level that I also have in my home country (currently Germany).

It's not a cheap country but 7k/month as "minimum income" is not a requirement out of necessity or kindness.