Other than the fact that it's hard to just put in a pocket and carry around until you see a bin, is it that smoking and littering are somewhat impulsive behaviours against better judgement? Someone doing one is inclined to do the other?
You can see other smokers doing it right when there's a bin nearby too, I've questioned them and never get an answer.
I think many people just live so thoughtlessly and carelessly, the rest of the city is dirty so why should they bother? It's really not much of an effort though.
Now your yard? That's just rude.