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.
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!