They also have more limited resources and tend to be glass cannons. D&D is a team game, I find that it works best when different PCs fill different roles. I don't see wizards as being the end-all be-all in games I play.
It's not that they are, either. I can't speak for anyone else's opinion, but when I talk about martial/caster disparity, it really comes down to variety of options and relative power of said options. There are many things a caster can do that can have very dramatic effects on the game. One spell slot can fundamentally change the nature of an encounter, adventure, or even campaign.
The only way to balance this is to pressure casters to get them to use more of their spell slots, but as they gain levels, each spell slot has increasingly more value, and ability to prevent them from having to expend as much energy to resolve scenarios.
The Barbarian and Fighter, on the flipside, has much fewer methods by which to alter the course of the game. Their damage potential certainly can end fights quickly by granting that most potent of status ailments...death....and it's certain that they have their place in the game.
But the game has taken away their truly incredible powers to alter the landscape of a world. No ability to raise a barbarian horde or an army of trained soldiers, unless the DM specifically gives them that power.
Whereas nobody has to specifically give a Wizard the power to create Simulacrums or create a fortress for himself. He has spells that do that. It doesn't help that spellcasting warriors like Paladins are just as effective as their non-spellcasting counterparts, or that WotC is content to let casters usurp more of what makes non-spellcasters stand out.
Many players are perfectly content dishing out hit point damage and turning enemies into mincemeat, and probably don't care what things spellcasters can do. Many spellcasters are equally content just tossing out fireballs and cones of cold to directly support the cause of "make enemies dead".
But spellcasters don't have to do this. They can use a spell slot to turn a tough to win encounter into "mopping up charmed foes who have no ability to fight back". Or avoid an encounter completely. Or multiple encounters. Or to change the course of mighty rivers or grab the MacGuffin and teleport away to a place of safety.
It's not about what
will the spellcaster do, but what they
could do, that is my concern. I don't think all this potential should be given strictly to those who wield spells, especially narrative potential.
When I talk about martial/caster disparity, I'm not saying that Fighters are irrelevant. I'm saying they should be
more relevant, and the fact that the game designers are perfectly content to let a party of four casters function better without a Fighter or Barbarian than a party of four Fighters and/or Barbarians (or Rogues, for that matter) strikes me as very odd.
But I know there are people who feel this is
working as intended, and it's fine for them to have that opinion. I don't understand it, that's all.