It's the way our brains work. Deep down it's more advantageous in terms of survival to quickly notice bigger while more smaller things tend to only come into play once we engage the less emotional higher regions of the brain. Look at it this way.
- Level 5 fighter & level 5 wizard, both have 16 in their primary stat. Fighter has a longsword & shield. Wizard has a generic d12 cantrip we are using to avoid "but damage type" complications that typically work against the d12 cantrips (poison & necrotic), using a generic cantrip instead of toll/poison spray improves the caster making it easier to show the fighter being left behind if that's happening.
- Fighter attacks twice for 4.5+3 for a total average of 7.5 each or an average total of 15.
- Wizard casts his d12 cantrip once against the same target & does 13,. 13 is 2 less than 15
- GM gives the fighter a +1 longsword, at level 6 the fighter bumps his strength to 18
- Fighter attacks twice for 4.5+4+1 for a total average of 9.5 each or an average total of 19.
- Wizard casts his d12 cantrip once against the same target & does 13,. 13 is 6 less than 19
- Level 8 fighter bumps strength to 20 Wizard bumps int to 18
- Fighter attacks twice for 4.5+5+1 for a total average of 10.5 each or an average total of 21.
- Wizard casts his d12 cantrip once against the same target & does 13,. 13 is 8 less than 23
- Level 11, fighter gets a third attack, wizard gets a third d12 on the generic cantrip
- Fighter attacks three times for 4.5+5+1 for a total average of 8.5 each or an average total of 31.5.
- Wizard casts his d12 cantrip once against the same target & does 19.5,. 19.5 is 12 less than 31.5
- I don't think the 4th attack at 20 & 4h cantrip die at 17 is relevant but it only continues to get worse
- Fun fact: if the wizard misses one attack with that cantrip, he does zero damage that round. If the fighter misses one he does 7.5/9.5/21 that round.
That matchup with a sword & board fighter using a one handed longsword should have been no contest or at least somewhat close, it's fr from it though. Do something like a greatsword & GWM things get silly fast
- Level 5 fighter & level 5 wizard, both have 16 in their primary stat. Fighter has a longsword & shield. Wizard has a generic d12 cantrip we are using to avoid "but damage type" complications that typically work against the d12 cantrips (poison & necrotic), using a generic cantrip instead of toll/poison spray improves the caster making it easier to show the fighter being left behind if that's happening.
- Fighter attacks twice for 7+3+10 for a total average of 20 each or an average total of 40.
- Wizard casts his d12 cantrip once against the same target & does 13,. 13 is 7 less than 40
- GM gives the fighter a +1 longsword, at level 6 the fighter bumps his strength to 18
- Fighter attacks twice for 7+4+1+10 for a total average of 22 each or an average total of 44.
- Wizard casts his d12 cantrip once against the same target & does 13,. 13 is 31 less than 44
- Level 8 fighter bumps strength to 20 Wizard bumps int to 18
- Fighter attacks twice for7+5+1+10 for a total average of 23 each or an average total of 21.
- Wizard casts his d12 cantrip once against the same target & does 13,. 13 is 33 less than 46
- Level 11, fighter gets a third attack, wizard gets a third d12 on the generic cantrip
- Fighter attacks three times for 7+5+1+10 for a total average of 23 each or an average total of 31.5.
- Wizard casts his d12 cantrip once against the same target & does 19.5,. 19.5 is 49.5 less than 69
- I still don't think the 4th attack at 20 & 4h cantrip die at 17 is relevant but it only continues to get worse
The divergence gets worse if the wizard is using a normal d6 d8 or d10 cantrip that actually exists. That gap even with the odds so heavily stacked in favor of the fighter by using a theoretical cantrip that does not exist is so great that even if the wizard gets off a "big" nuke or manages to nuke a bunch of things to line up with dmg249's assumptions he's still lagging far behind.
"but why stick to damage only? aren't a lot of the wizard's best spells nondamage?" is the obvious question & I agree that they are. It has two big problems still. All of those spells are pegged to be roughly on par with martials using no feats& no magic weapons but that's a silly situation that really only occurs at low levels where resist nonmagic b/p/s is unusual & games where the GM explicitly chooses to use some kind of screw the martial monster(s) they have not provided a way to bypass it. There are basically no feats that improve a caster's ability to cast spells (nondamage ones especially) in any fashion even close to the dramatic step up as something like that longword+shield to greatsword+GWM. Factor on top of all that the fact that any leveled spell consumes a resource(spell slot) while nearly all of those nondamage spells tend towards being concentration & the gap widens even before considering the frequency of save to nullify then free save every round to dismiss on those spells.
Some of this could have been avoided by giving more concentration spells a hex-like move the spell as a bonus action, using concentration less, spells with better power, less costly scaling of spells, or not enacting such an excessive number of safeguards working in parallel against the LFQW of past editions.