But it has always being the hard way.
Magic-users need to spend years of study to learn how to cast spells, and constant focus on their arcane studies during their career to become a successful wizard.
They need the spellbook that requires special inks and that means of alot of investments in gold and time. And if he loses his spellbok he is pretty much screwed.
His spells can be disrupted, they can be resisted, the damage they inflict can be halved or be the minimum possible. The M-U might find himself without the proper spell he needs in the ocasion, and he needs to cast them wisely, otherwise he won't have them when he really needs them.
Magic in D&D is unpredictable, limited and mostly unreliable, and the magic-user needs to handle that every day.
Fighters just have a much easier and simpler life.