In friends it's explained by Monica inheriting the apartment or something other

The huge apartments are kind of an acceptable break from reality for me. Yeah they occasionally have in-universe handwaves but the real reason is it's a set and it would be hard to film in anything smaller, especially in a multi camera sitcom like Friends.

I didn't watch enough Sex and the City to see if they had an explanation for everyone's wealth, but it was pretty clear that all the characters were wealthy.

You have to consider they only have 2 or 3 walls, and usually no ceiling. Very rarely do they ever acknowledge the Fourth wall.

i like in fresh prince when will wonders that if they are so rich how come they cant afford a ceiling

I have nothing to support this but I’ve always thought it was to play into middle America’s biased perception of New York/LA.

It always seems like middle Americans thought about the increased salaries in these areas with taking the CoL there into consideration. They’d think someone making huge salaries must have all this extra income without considering they’re probably paying 2-3x the cost for housing alone, if not more.