Instead of adding Optifine, they should make the game not a laggy piece of stuff.
That is exactly what they are doing, actually. As said before, Optifine people didn't want to help applying what they do to vanilla, so instead they are just doing as many fixes as possible based off what they know of Optifine. It is like, indirectly trying to be Optifine.
Optifine does a lot of changes, more than could be documented, so they would need hand-holding to get that into vanilla. At the same time, not everything from Optifine can be applied as it could contradict some basic features. That is probably why they didn't want to help.