They don't have to. They do so because they enjoy doing it. You're being a bit dramatic.
They don't. But being a hugely successful game with a huge loyal fan base it would make sense to continue support for the game. Unless you are unaware they made over a million sales of the horse armour dlc which sold for $0.99 each. So over a million dollars on one dlc and more of an incentive to release more patches. Not to mention all the other dlcs they've released afterwards as well as another expansion.