It just does. People can spay or neuter their animals. It's not hard. It's very easy, in fact, and anyway, these people have made a decision to take care of an animal. But owners often don't.
People are stupid and lazy and bad.
Perhaps I am just a softy. I probably am. But the above has been bothering me very much. I know I cannot take the dog, for numerous reasons, but I didn't want my friend to give up until he found a home. I didn't want the pound to kill it by mistake or the vet to recommend it being put down or the rescue organization being to taxed already with wards.