Fighters never get the knowledge to know interesting things.
This isn't a Fighter-only problem. While the Fighter does have a rather anemic skill list, Backgrounds exist that allow someone to gain any skill they care to. While knowledge skills are usually Int or Wis based, Fighters are far from the only classes that do not have a focus in Int or Wis.
Barbarians, Bards*, Paladins, Rogues*, Sorcerers, and Warlocks are all classes that generally rate Int or Wis below Constitution as ability score priorities.
Again, of course, a player can choose to build their character to have those priorities, but the PHB has build advice as I recall, and does not advise them to do so.
*Bards and Rogues do have abilities that can give them good skill checks in things they aren't particularly good at, however. Rogues do have proficiency in Int saves, so you might think Int is actually a secondary attribute for them, but beyond possibly Investigation checks, I'm fairly confident you could be a Rogue with an Int of 8 and not even notice.
Much has been said earlier about the Fighter's amazing ability to use Second Wind to be good at ability checks- it's a nice clutch ability, but it's not one without cost! Not only do you only have one use for a long time, it's not like the game is going to always let you take an hour long nap between skill checks! Further, when (not if) combat breaks out, that surgeless healing is quite useful (I've been playing Tales of the Valiant which got rid of Second Wind, and suddenly I've noticed how useful it really was!), granting some measure of additional resistance to harm.
Sure, you could possibly make a really amazing check every so often, but that's not really enough to call a Fighter a "skills character".
That all having been said, D&D is meant to be a team game. Not every character is going to be amazing at all times. Unfortunately, the PHB doesn't really get into the weeds of what I like to call "party optimization". A group should ensure that all bases are covered, beyond the bare bones "who is going to be the healer" that is about as far as most groups get.
That the Fighter is generally relegated to "fight stuff" and "brute skills" is an unfortunate baseline, but the generic identity of the Fighter can actually be a strength in this situation. You can build your Fighter to run off of either Str or Dex, with a Con of 14 or so and be perfectly free to round them out any way you want, because there is almost no pressure to do anything else (beyond having maybe not dumping Wis, or deciding to attend Community Wizard College as an Eldritch Knight).
In theory, you could use your bonus Feats to further develop your secondary interests, but you don't get a lot of these, and having to dip into the same reserve for ability improvements, combat power, lasting power, and out of combat ability is a particularly dumb design decision and has been for some time- the typical D&D play loop has become one where combat isn't a "fail state", it's an inevitability. As a result, it simply makes sense to use your limited Feat choices to be better at winning (and thereby surviving) combats.
Arguments could be made that you could avoid combat with a good skill roll, and that's true, but the d20 is so swingy that even high ability+double proficiency bonus isn't any guarantee, and if you bothered to acquire these and they fail you, you'll probably wish you had bulked up your resilience and performance in other ways.
In the end, the Fighter has a singular purpose, but whether it's a straightjacket or a life preserver is a matter of point of view. Some players don't want to engage in the game beyond such a limited role, and are probably happy that they won't feel pressured into another one!
For example, some time ago, I played with a group that included a Paladin, and no other high Charisma characters. When social encounters came up, we would often ask him to be our party spokesperson, a role he really didn't seem to want, and often tried to duck! If you'd offered him some sort of alt-Paladin that used Wisdom instead of Charisma (there was a Paladin archetype in Pathfinder 1e for Dwarves like this, as I recall), he'd likely have been much happier for it!