a level 20 wizard has 2 1st or 2nd level spells at will (including action denied control like tasha's laugh, instant miss like shield, that new one that gives disadvantage kinda, even just detect magic) a few dozen effects they can throw around 1st-4th level, a handful of 5th and 6th level major game changes (these alone if nothing else are heads and toes over a fighter) and 3 major reality altering spellsAt level 1:
It's not that hard to argue that the level 1 characters are balanced with each other. It's possible but very hard to argue that the level 20 characters are balanced. It's ridiculous to argue that both are.
- A fighter has a basic speed of 30ft walking and in combat swings their sharpened piece of metal hard and fast at a target within its physical reach
- A wizard has a basic speed of 30ft walking and in combat can cast Burning Hands a couple of times per day.
- At level 20:
- A fighter has a basic speed of 30ft walking and in combat swings their magically fast piece of metal harder and faster at a target within its physical reach
- A wizard has possibly True Polymorphed itself permanently into an adult red dragon, which means they fly and can fire breath one round in three - and everything else they can do.
they also have cantrips that although slightly below an average fighters damage (and very below an optimized one) still let them at will hit for some damage.
all of that is before subclass/feat/items.
someone up thread said they can't heal, but vampiric touch, wither and bloom, and wish sure can.
clerics of 1/2 the domains and bards of at least 2 colleges can do all of that with good AC and weapons...
edit 1st and 2nd level spells mean the healing wither and bloom and hold person are on the table... how could I forget