Those mods are developed by lots of different people and there's a pool of hundreds or thousands of mod developers. You can't compare that output to a single team. Everything in the base game also has to not break backwards compatibility or balance, or need to be removed later. They also spend time developing complex mobs like that new bouncy thing, and making sure they don't have major bugs or performance issues (since they interact with everything else in the game). The team also develops the game systems and rendering and so on. They've done big overhauls to the chunk format multiple times, including increasing the world height, completely redoing how liquids are stored, more flexible block storage, smoothing out new terrain generation after updates, etc.. While keeping backwards compatibility to allow worlds to be incrementally updated to the new format.

And anything in the base game needs to work for 100% of game installs and servers. Mods are only installed by a small fraction of users (and they often have all sorts of bugs).

A modder could get 90% of the way to something worrhy of being in the bade gsme. It's the last 10% that's hard.