The performance improvements are real, for some people. Yet Optifine causes issues for others. It also requires a fair bit of configuration to get optimal.
Mojang needs to keep things simple, functional and consistent for the vast majority of their players. They choose the best defaults that work for the most number of people.
Also, modded players will let you know just how badly Optifine breaks rendering calls. The first step of any troubleshooting is checking if Optifine is installed.
