The reason is to preserve compatibility for everyone's configuration that they carefully built up over decades. It's quite hard to change things to enable some new thing without annoying many users. The last time that was tried was in the '90s and there are still zsh developers around who experienced the fallout. I value the fact that my shell doesn't change on me without warning like many GUI desktop environments.

In recent years the existence of frameworks like oh-my-zsh has contributed further to this situation because it has separated the detailed configuration from the base zsh project. It's a pity omz doesn't enable more really. Many things it does are not really useful unless you take the time to actually learn them. A lot of plugins just define aliases but if you don't know what they are, they just sit there unused.