That only holds true for very low level monsters. What if we're talking about a 20th level mage fighting a bunch of 7th level creatures? Many level 7 monsters have well over 100 hp. A fireball that does on average 21 damage isn't going to help much. Even a 9th level meteor swarm only does 42 on average. See the problem? Yeah, a 5th+ level wizard can wipe out a bunch of 1st level kobolds, which are 4 levels below him. But the same is not at all true of higher level wizards fighting creatures with far greater level gaps. This is because the scaling of hp far outpaces the scaling of spell damage.
The game goes from one extreme to the other. At 1st level, even a level 1 spell is incredibly deadly. When characters often have single digit hp, or low teens at the most, a 3d4 +3 damage magic missile or 4d8 sleep is very likely to one-shot just about any character. But then, as you go up in level, the situation quickly reverses itself, as hp quickly multiply but spell damage increases very little. I just want things to be consistent, and for blasting spells to not become far less useful than save-or-suck spells.