And because of the TV rule that every protagonist home must be unrealistically spacious and well-appointed compared to their stated income bracket and other factors. Unless the protagonist is canonically rich af, then they just play it straight.
The million dollar idea! Smell-o-sense TV! I still stand by the my statement that the Ninja Turtle's home looked cozy, I can't speak to the aroma. Depending on the iteration of the turtles, sometimes it's more of an abandoned subway station and not a raw sewage system.
They wore trenchcoats and fedoras to pass as human. They didn't have to live in the sewer. Humans were dumb as shit if they could be so easily fooled by just coats and hats that clearly still had their giant turtle heads exposed!
I like the "splinter was a human" versions better. Despite everything absurd in TMNT, it's how easily he just learns ninjitsu that seems impossible to me. In one version he learns it from a single book in the sewers!