Fireball works for 8d6 on a single target, imagine a 15th level Evoker with Elemental Adept casting Fireball when there are two or more enemies and Firebolt when their aren't. Are you telling me the Fighter will out-damage them? 4 3 3 3 2 1 1 are the Wizard's spell slots. That is 3x8d6, 3x9d6, 2x10d6, 1x11d6, 1x12d6 whenever there are two or more enemies together and that Wizard has a VERY high save (8 + 5 + 5 = 18 DC at least). 24 + 27 + 20 + 11 + 12 = 93d6 x 2 enemies = 186d6 possible damage just from Fireball. Spice with Firebolt for probably another 20x3d10+5 as you like. Also they can be 150 feet from the target.
10 rounds of Fireball, 20 or so rounds of Firebolt for a possible : 186d6 + 60d10 + 100 damage.
Your fighter can do that?
So, you are assuming that the enemies fail all saves vs the fireballs, and that you never miss with with the firebolts? Plus you never have a fire immune enemy, and that none of the enemies are capable of casting counterspell?
Assuming average rolls, and assuming you crit on 1 in 20 attacks, that means you critted on one of those firebolts for another 16 points. Total damage: your wizard dealt out 1,096 damage.
OK, same general assumptions for a lvl 15 fighter - they have a feat (since you have elemental adept), they never miss, and none of their opponents have damage resistance, and they have 30 rounds worth of attacks.
level 15, 20 str, Greatsword, great weapon style (not that it matters with average damage), and for the feat, Great Weapon Master. Damage is 2d6+20 per hit with a non magical greats sword. (or 4d6+22 if they have the legendary weapon Haziwran.

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3 attacks per round, x30 rounds, +3 more attacks with actions surge = 93 attacks. Great weapon master gives a bonus action attack on a crit, assume he crits 1 in 20 rolls, so 4.65 crits, meaning another 4 attacks. Total attack rolls: 97
Average damage per attack: 22 x97 = 2,134 damage. Plus 8d6 for the four crits, so another 28 points of damage. Total: 2,162 damage
This gives us:
Wizard: 1,096 total damage to single targets. 651 damage to groups of 2+ from fireball, the rest is single target fire bolt damage.
Fighter: 2,162 damage to single targets. Or 651 damage to three different targets. If you take away the +10 damage from Great Weapon master, they still do 1,192 damage.
The number goes up if you assume the fighter has a magic sword with bonus damage (or some other way of getting bonus damage dice). Haziwran is a +2 greatsword that does +2d6 necrotic damage. If the fighter had that, they'd do an +5.5 damage per hit on average, for another 533 damage.
I haven't even added in potential damage increases from sub-class features (Champion critting on a 19-20, Battlemaster using Superiority dice, Eldritch Knight using Haste and Fireball). You did add in the +5 damage per firebolt for being a level 10 evoker wizard.
So yeah, a fighter can do that.
Wizards are best at battlefield control and doing AoE damage to clear out groups of lower HP creatures. Plus lots of utility spells.
Fighters are better at standing toe-to-toe with large bags of hit points and whittling them down. Plus various other tricks from feats and sub-class abilities.
Neither of them suck.