If it's addressed at all, its usually handwaved away as either:

• Friends Rent Control (Warning: TVTropes)
• Actually not as large as it seems on TV.

How I Met Your Mother did the latter pretty well. The whole show was a story the narrator was telling his kids, so it was all based off of how he remembered his life at the time. What we see on TV is just him remembering the NYC apartment bigger than it was.

Some of that is for shooting. Like on The Fresh Prince Of Bel-Air, Uncle Phil can definitely afford a house that big, but its layout is bizarre. The downstairs is dominated by a living room and kitchen in a sort of L shape that does not match any exterior shot and is a frankly mediocre use of space. It's because there's three cameras roving around where half the walls should be. And a live studio audience! They have to see and hear what's going on, or else it might as well be a laugh track.

Basically - it's a stage. It's not a set. You're watching a briskly-edited play, with an unusually high sense of verisimilitude. So yes, if you map out Jerry Seinfeld's apartment, it's deep and spacious and maybe doesn't connect to the hallway right. But that's just to give the actors somewhere to move. On-camera, it looks compressed, with very little of that floor space visible, and the back office simply left as a hand-wave for whatever you think is missing.

Yeah the poor students living in manhattan etc.

Whats insane is we used to. My janitor dad and homemaker mom afforded a five bedroom, three bathroom house with full basement, workshop, seperate dining room, living room, breakfast nook, kitchen. it was dillapitated sure but not to an unlivable level. Massive porch.