What is it exactly that makes Wizards OP?

[Khanderas]

20 Fighters vs 20 other Fighters are about equal.

20 Fighters vs 1 other Fighter means certain doom to the outnumbered one.

20 Fighters vs 1 full caster prepared spells doom for the 20 Fighters.
(This is at higher levels obviously, where everyone mentioned has about the same level)

(Celerity-timestop, port out of melee range if he was, else, fly, cloudkill, invisible walls (cut off / box in say 5 of them to kill first), greater invisibility, summon spells to distract and finish with a handful of save or lose spells. If any alive, summon a few more monsters and start evocating. If still alive and almost out of spells, teleport home, have a nice dinner, sleep, refresh all spells and scry out the survivors).

And therewego. Why a caster wins even on empty-field-of-battle that is argurable the Fighters home turf. If we go into political pressure (having him arrested / executed for fake charges), Assassination 1 on 1 (Scry, teleport, ???, win) or nonlethal dickery (ooh the options), the Fighter is at a disadvantage from the start.