That makes little sense though. Because he didn't pick it to "learn something new" but as his role in the party. Abandoning getting actual respect, decent treatment, and recognition in favor of adding being an errand boy on top of his other skills.
Dude, you're missing the point entirely.
MC specifically states on page 12: "I'm the only one in my party that lacks
basic knowledge, after all."
Remember: he only
just started dungeon diving. He does not yet have the
foundational knowledge that others already have. Which is part of why he never realized that he was becoming so stupidly OP in the first place - he had no point of reference, not even the basic common sense of other dungeoneers, that would have helped him to realize what was going on. He had to get clubbed over the head with clue-by-fours by his two ladies - repeatedly even - before it finally began to sink in. It isn't that he's dumb. It's because he had
absolutely no frame of reference!
His very first dungeon dive was basically like being dropped into Dark Souls on "NG+7" difficulty without anyone or anything telling him so, and because he did not have the common knowledge of other dungeoneers, he thought at first that it was "Normal" to have that kind of experience.
Also remember that MC does NOT have your perspective as a reader. He can only see what he sees. He does not have your "big picture" POV.
As for why his lack of basic knowledge is such a problem?
You can ask
anyone who plays RPG's as a hobby: even if you get twinked (see:
Twinking on Wikipedia) on your very first playthrough and have all the skills in the game instantly granted to you, you're still not gonna know what the hell to do with them. That's where having guides and walk-throughs are helpful.
And that's what he's doing right now. He's got these cheat-level skills, and they're great. But he knows that he lacks the common knowledge that will let him get the most out of those skills. He can literally turn monsters into pink mist, which is fine if he was soloing... But in a party? He needs to be more precise so there'll be good materials left for the rest of his party.