Because they're cunts 
I've been on country walks where the path has numerous bags of dog shit as if people think it's OK to still leave it lying around if it's in a bag, so not only do you have the dog shit but also discarded plastic 
It's probably worse in a bag than not in a bag as the weather and things like slugs won't get to it to break it down so it hangs around for an ice age 
I have to agree about them being cunts - I always pick up my dog's crap, and put it in a bin.
The bags are bio degradable, but yes, they can hang around.
I recently went to Sandringham and they have a "stick and flick" policy (use a stick to fflick the crap into the undergrowth)in their parkland - no bags at all.