Because of time and effort.
Most mods are noob mods that either only change stats and or graphics or copied code from other mods so they are unable to do things themselves let alone updating by themselves. Cosmoteer changing code and mechanics with nearly every release doesn't help either. True modding requires effort which most random modders don't really bring along.
As for the rest it's simply time and life. You do modding for free so nobody has any obligation.
