Because there are other features for most casters that aren't just spells.
But, we did nerf bards to make them half-casters. At one point Sorcerers were subclasses of Wizards and Warlocks were a subclass of Clerics, as well.
I love the full-caster Bard. Its mythological accuracy is significant.
Half-casters are an appealing setting approach.
For low-magic mages, the Paladin makes a great Tolkien Gandalf concept.
The Trickster makes a great "magician" concept.
The Ranger makes a great nature magic concept. Barbarian too.
The Monk makes a great East-Asian-esque concept.
I would reserve Eldritch Knight for a high elf culture specifically.
Psi Knight is awesome for a telekinetic concept.
The Artificer covers many concepts, including magic item creator, potion brewer, alchemist, and perhaps in the future a necromancer with an undead "pet".
5e has excellent options for this lower magic mood.
Anyway... back to the Fighter.