Because then they have to spend money to keep things up better. Seems like a you sxratch my itch, I'll scratch yours. Maybe if warranted, call the health Dept., or what about the NAT squad or a tv crew? Take them to court if you do not get your deposit and you feel you are in the right.
Sometimes I think that the people who do not take care of things probably cost the landlord a ton of money, and the value of their property goes way down and so does their borrowing power. Sometimes it is a no win for everyone.