Why is it, toothpaste clings to the inside of my drains?

It's not like anyone brushes their teeth, without "wasting" water.
The drains begin to drain slow, then end up stinking.

I was talking to some neighbors, they say they have the same problem.

The drains are nasty, this is pvc pipe. I've cleaned them before, the
stench is overwhelming.

Maybe someone is ****ing in my drains, I dunno, but its ****ing me off.