For all the obvious reasons, of course.
Urine for one. Both the smell of urine and the puddles of urine behind the cement columns from years of outdoor bathroom use. Overpasses are drenched in urine.
Overpasses are drafty. Caulking and generous use of weather stripping be damned.
Overpasses also typically team with flying rats, otherwise known as pigeons. There was that Home Alone with the indigent woman who was a scary character at first but as the show went on +Macaulay Culkin learned that she actually had a heart of gold? She just happened to like birds and living in +Central Park?
When the movie first came out, as a person in the theater, it wasn't hard for me to accept that Culkin developed a strong attachment to the woman.
In reality? That poor woman would have smelled a lot like bird poop. And she probably only had a few more months to live on account of having the hantavirus. I'm pretty sure you can catch that from pigeons (note my reference to flying rodents above).
I'm just saying.
Nothing funny in a timeless Christmas story about child neglect if it ends with the child dying from the hantavirus.
I could go on.
But the real reason I would never want to live under an overpass is that living under an overpass sounds dangerously close to the start of a pun. And as everyone knows, liberal use of puns is a predictor in children for a predilection towards serial killing.
To each his own though. You want to live under an overpass? Go for it.
Living under an overpass . . . eh.