In 1-2 a level 20 fighter with the 20th level right equipment rules the game. Hand of Vecna + Blackrazor + Sword of Kas (dual wielding) + Ring of regeneration + Full Plate +5 + Amulet of Magic Resistance will beat in any other build. The way to make an interesting fighter is to give them interesting equipment.
You are assuming that the Wizard is a evoker trying to "throw bigger numbers". That is NOT how a high INT wizard would act. He would first, cast mordekainen's disjunction. All OP gear from the fighter will become mundane. And you are comparing a fighter with the best gear vs a "mundane" lv 20 mage which is his journey to lv 20 probably crafted a lot of powerful magical items.
As for attacks of opportunity, the mage in question if he is smart, he will avoid being hit.
Again, that's well argued but I still get stuck where the rubber meets the road -- I just have more fun playing unbalanced games. BG2 and DOS2 were just more fun than NWN2, IWD2 or PoE1 (combat only -- everything in PoE was great, except combat)
Every amazing RPG which I played is extremely unbalanced. I don't know a single RPG which is balanced and I loved.