First of all, as some people have said, Notch could just abandon the game and go buy himself an island in Hawaii.
Secondly, that's just the way mods work. If a game is moddable, modders will add things the developer didn't. And if the developer start going around adding all of the good mods, the game would quickly become unrecognizable, and there will STILL be more mods with amazing things the game doesn't have.
Finally, to the people saying that the updates are to few and far in between, as a modder I have to update my mod every single time Minecraft updates. Frankly, it's a pain in the ass, and my mod isn't even nearly as large or complicated as some of the mods available. I can only pity the people who must have pages upon pages of code to update, and Notch knows this. Perhaps if the modding community were smaller or non-existent, he could release updates every few days with constant minor updates. But if he were to do that with the modding community as big as it is, he might lose their support, and since mods are unquestionably one of the reasons the game is so popular, it wouldn't be the smartest move possible. So rather than do that, he releases more significant updates, but less frequently.