And each one used causes the wizard's ability in combat to fall. With no spells used outside of combat, the wizard can only cast spells in half(2) the rounds of combat in an adventuring day. If he uses 6-8 spells outside of combat, he's down to about 1 in each combat. Meanwhile the fighter is trucking away with 4 attacks a round and 2 action surges in most of the combats.
IF, and its a BIG IF, you ascribe to the 6-8 encounters per day metric. I see a lot of comments on these boards, and from folks with a problem with the wizard's power, saying that THEY DON'T USE 6-8 encounters per adventuring day for a variety of reasons - time it takes for combat, pacing of their game, journeying, etc.
Unfortunately, I don't think there is any way to balance either of these classes with people approaching from two different premises.
In my last game, a sandbox, an encounter or two max per day usually, the spellcasters dominated. In fact, the only Martial was a poorly run Arcane Archer (arguably weak to begin with). But his being there that week, or his missing the session made zero difference in the outcome of the fights. Even when I upped the DCs of the enemies (double +), or threw higher level "PC" spellcasters at the party, zero, zip, no effect.
Casters were able to combine every round damage cantrips (scaling) for ongoing damage (and saving spell slots!), the occasional big flashy spell (Evokers in the house!) to eliminate henchmen and blast biggies, AOE effects from clerics (spiritual guardians, twilight stuff, etc.) to prevent melee NPCs from closing (or to prevent them from getting more than a round in before the passive damage did them in), to nuking the BBG or the biggest perceived threat with the big flashies, while protecting themselves with perfect prep (invisible imp scouts, darkness/devilsight protections, Twilight temp hps, etc.).
And sure, I, as DM, could have easily wiped the party any time I wanted to, but keeping it "realistic" in terms of encounters - I couldn't always start my enemy casters in dispel magic or counterspell range EVERY SINGLE ENCOUNTER (which were 2-3 a day, to 1 a day, to none for weeks). And when they weren't in combat, the Zone of Truths, Charm Persons, Scry, Light spells out the wazoo, etc. made the mundane adventuring (social, exploration) pretty pointless. We got to the point where we just skipped large fights because there was zero reason to waste game time going through the motions of what would be a clear mop up.
Oh, and all the characters had about the same range of HP when we stopped - 40-46, regardless of class. So it wasn't even a matter of the casters being "squishier" than the Martial we had.
I've come to the realization, that its just the way the game is now. And if I want more "mundane", I have to look at using Sidekicks as primary PC's, curating spell lists, or maybe just having Warlocks be the only type of caster out there (covering clerical, magical, sorcerous, etc.).