Because landlords are in business to make money, plain & simple. A number of landlords today are publicly traded companies, so they cater to their shareholders first & foremost, for whom they have a legal responsibility to, as prescribed by Federal Law. When businesses give to charities, donates water to disaster victims, or otherwise, it's because its either good marketing for them to get more people in their doors or they are getting a big tax incentive from it. Landlords aren't typically in need of marketing themselves, so that leaves only tax incentives, so unless they get an incentive from somebody, it's usually not going to happen.
As American editorialist, Ambrose Bierce, once said: