i dont believe in chem trails but why so many planes fly over the cities

[CWMV]

Eh our species hasnt been around too long, a few million years is still new kid on the block in geologic time.

AS to the OP, well, I mean that's where people go, to cities. Very few flights going to the middle of nowhere.
And why would common sense dictate that they avoid overflying cities? What am I missing?