This depends entirely on the individual players and their personalities and desires. One player sitting out of 80% of the game can be very good or even ideal on certain tables with certain players and usually when this happens it has nothing to do with mechanics.
Sure, you might have a player who can't make whole sessions, or who wants to sit out certain sorts of play. And, it probably has nothing to do with mechanics.
But, in D&D, if you don't want to play a spellcaster, the mechanics force you into specializing not just in combat, but in single-target DPR, as you only viable contribution to the party's success.
The mythical 'wake me when the combat starts' player can thus choose Champion Fighter. He can also choose any caster, and just load up on combat spells.
But, if you're any other sort of player, and your concept doesn't call for spellcasting, you're forced into that.
I get that, but 5E is flexible enough that no character's role is defined or locked in by their class. This includes the fighter class.
That is extremely true of full casters, with only a couple of them nominally locked out of healing, and true enough, I guess, of half-/third-casters and others with supernatural powers that have significant non-combat applications.
When it comes to the handful of benighted sub-classes that don't use spells at all, tho, they just happen to be locked into single-target DPR.
Subclass, backgrounds, skills, feats, races and ability score distribution all heavily influence what your character can and can't do mechanically
Meh. That's down to checks, which, between bounded accuracy and reliance on arbitrary DM calls, doesn't amount to much, and which, when comparing classes, let alone sub-classes, is downright meaningless, as they aren't class features, but things literally any class can take.
Would it be okay for the wizard only to have evocation spells and tell them to "take a subclass, skill prof, feat, race and ability score mod to do anything else"?
The answer is no.
So why is it okay for the more popular fighter?