I mean how can they?
I have 2 cats and now 1 puppy. I watch my puppy when she is out and know where she is, I can see if she is pooing. If she disappears under a bush I check (with my sense of smell) and pick up if necessary. I carry poo bags everywhere.
However, my cats come and go as they please. When they were kittens I had a litter tray that they used, but as they started to go outside with a cat flap, they stopped using the litter tray.
My (ex) neighbour asked me to come and pick up after my cats, but as there are lots and lots of cats in the area, there was no knowing that it was my cats.
I am not a bad pet owner. But I think it is unreasonable to expect cat owners to have the same poo pick up responsibility as dog owners.