The game is also too easy with all three crafts perked up, and this imbalance is greeted with "just don't use all three crafts."
So again, why not have the option for people who want to use it?
Because it would make the game even more insane.
Example - I go make 1000 iron daggers (or whatever the number is) and get my smithing up to 100, taking all the smithing perks as I go. I then make enough health pots to level alchemy to 100, taking those perks as I go. I enchant those 1000 iron daggers to get enchanting to 100, taking all those perks as I go.
I now go make my character legendary enchanted armor, weapons, accessories, etc.
Now I re-spec that character, putting all those previously earned perks into stealth, archery, light armor (or heavy armor, one handed, you get the point).
As bad as it is now when you grind smithing etc. to overpower yourself, you'd be able to do it all over again to the third degree. In fact, the only complaint I've seen from people who grind out the crating to level is that their combat skills didn't keep up because enemies leveled, and these people have no combat skills. Well, re-spec would fix all that for them.
And that would be awful, IMO.