There's nothing inherently un-fun about being a high level Fighter. You take the hits, you deal the damage, you can play out several fantasies, from the visceral, primal satisfaction of beating someone up with a sword the size of Texas to being a cunning archer, to being a savvy master of tactical combat, and so on.
What is unfun is when the game shifts to requiring you to deal with "weird enemy supernatural ability #483" and you find yourself needing magic to overcome it. What is unfun is when things you struggle with, the magical classes have answers for. What is unfun is when the abilities of other classes are more impactful than what you are given. Like, take Indomitable, which lets you reroll a failed save (which you were probably unlikely to succeed at in the first place, since you can, at best, have 3 good saves), and Mr. Paladin over there gets to add his Charisma bonus to all saves no matter where he is (but for you to get that benefit, you have to be glued to him).
Or how you have no ability to make your attacks more accurate (outside of subclass), but every Barbarian can just give themselves advantage whenever they want, the casters are tossing d4's, d6's, and rerolls around like candy, and the Rogue, of all people, gets the power to say "nah, I hit brah"!
If a Fighter gets grappled, he has to hope he has good Athletics and luck. If a caster gets grappled, hey, they might be able to Misty Step away!
If a melee Fighter has to fight a flying dragon, he has to use a bow or toss javelins. A caster could have powerful spells, or even the ability to leap into the air or fly.
If a Fighter gets poisoned, paralyzed, petrified, or any other of nasty occurrences that he is probably the prime target for being on the receiving end of, he can't do much about it. But a spell can be cast to make that all go away.
By this point, I can tell some people are ready to angrily reply to me on this, so let me be clear. I'm not saying playing a non-supernatural character isn't fun. But D&D is built to make playing a supernatural character potentially more fun, or at the very least, to have abilities to handle the various circumstances the game throws at a character that the non-supernatural character can only dream of.
Which can lead to a miserable experience. I stress the word "can" here because I know, someone is going to say "I've never seen any of this in any game I have ever played or heard of" or "obviously the DM has to do X, Y, or Z to make sure this never, ever happens" or even "working as intended" (lol).
But this is a reality some people do have to deal with, I've seen it a lot, and it's a consequence of the fact that, from the very beginning, D&D's approach to magic has been pretty much "magic can do anything", while trying to keep people without magic more or less grounded when it comes to their abilities.
