Fireball does an average of 28 damage per target.
It does an average of 28 damage per target that fails their save and 14 per target that make their save, and keep in mind that this is a dexterity saving throw, so if you are hitting more than one enemy some will almost always get a +2 cover bonus on their save and a +5 cover bonus is not uncommon RAW.
20 per target is an estimate when considering those factors and I think it is fairly accurate estimate. I prefaced it with "about" and it may 1 or 2 off, but the number certainly isn't 28 per target or anything close to that.
~50 damage per Fireball is the floor of the spell and not the ceiling, because you wouldn't cast Fireball if you can't catch multiple targets.
Then you would not heavily damage them either and the fighter would outrun you by even more
From my experience, you'll have plenty of opportunities to hit at least three.
It will happen, but not twice on most adventuring days.
Even then, you're comparing the fighter and caster at one single level where both of them gain a power spike with Extra Attack and 3rd level spells respectively.
What about the stretch of levels after that, where the caster continues to ramp up with additional spell slots? The fighter only gains a third attack at level 11.
And a 4th attack and 20th level, and subclass abilities, and extra action surge, and legendary saves are starting to be more prevalent.
In terms of damage a fighter is typically going to outrun a Wizard over the course of a day at any level as long as he or she has a magic weapon. So will a Monk, Paladin, Ranger, most melee Clerics and Barbarians.
That does not mean the fighter is more powerful, far from it, but she does do more damage.