Maxperson
Morkus from Orkus
There are only 2 in the PHB. Power Word Kill, which is useless in most high level fights until multiple rounds have passed and the Wizard hopes that he's not wasting it on a creature with more than 100 hit points left. And Meteor Swarm which does 70-140 damage as a 1 use special ability like action surge. The fighter using action surge(comparable high level 1 use power) hits(probably 3 out of 4 swings) for 74 damage + any crits + any superiority dice + possible affects from maneuvers + some amount of additional damage from re-rolled 1s and 2s.Actually that's that not the problem. HP is fine.
Here's the thing, if you look at the damage of a 1st level spell and a 9th level spell...
Most high CR creatures are solitary, so the area affect of the meteor swarm only serves to kill party members caught in the blast area, because the wizard is unlikely to be an evoker. It might also clear out some mooks, but that's pretty irrelevant.
Between base damage, crits, superiority dice, and re-rolled 1s and 2s, the fighter with that many attacks would kill or nearly kill most CR 20 creatures in a single round where they action surge. That's absurd.The 20th level fighter should have 6-10 attacks BASE with over 15 attacks with action Surge.