I think this is partially just observational bias.

You only notice the smokers littering and not the ones squeezing the cherry out and pocketing the filter or throwing it away.

A lot of smokers do the above and if you ever meet them they smell next level disgusting. Their car ash tray will be full of butts, and there car will reek. They often just skoke inside, and their entire living space smells of stale tobacco. Their pants will reek because they store them in their pocket until they find a bin. But they don’t litter. The people who are trying to avoid smelling like an ash tray are the ones littering.