Magic drains mana sooner than mélêè gains fatigue.
Also, spells have tended to be more nuanced in designs, and thus casting them requires more than simply wailing on an enemy with your weapon. However, this is not always exclusive of physical attacks. In games like Mortal Kombat or Tekken, et al, you have combos and such. WoW has special so-called combo points for Rogues. Rift has the whatchamacallits too, but their classes are much more articulate.
So, what would autocasting do to play?
If the experience system gets improved, repetitive practice would not be so important viz. no more grinding (except to produce goods).