The primary reason is that there's no money in it. Like movies, your "one shot" game (without some sort of continuous billing e.g. mmo, subscription, continuous stream of DLCs) makes most of its revenue in the first few weeks, and once the kinks are ironed out what it makes afterwards doesn't really depend on maintenance.
Additional maintenance doesn't pay for itself, the producer doesn't pay the devs for that, and thus the devs take on the next contract to pay the bills. Not to mention additional maintenance is a risk.