The starters would be insanely lopsided. There isn't supposed to be one clear winner out of the starters outside of the story. Each starter is catered towards specific strengths and , imo, are supposed to balance each other out. Like Froakie is fast, Delphox hits hard (on the special side specifically), and Chespin is a hard-hitting tank. If these starters got their hidden abilities, Froakie would edge out immensely with STAB 100% of the time.
I am all for having an item that lets you switch from regular ability to hidden ability, but only in post-game.