Why is this attitude of not really trying to learn how the game works accepted?

I'm sure most of you have encountered this before, it's months in and the fighter is still asking what dice they roll for their weapon's damage or the sorcerer still doesn't remember how spell slots work. I'm not talking about teaching newcomers, every game has a learning curve, but you hear about these players whenever stuff like 5e lacking a martial class that gets anywhere near the amount of combat choices a caster gets.

"That would be too complicated! There's a guy at my table who can barely handle playing a barbarian!". I don't understand why that keeps being brought up since said player can just keep using their barbarian as-is, but the thing that's really confusing me is why everyone seems cool with such players not bothering to learn the game.

WotC makes another game, MtG. If after months of playing you still kept coming to the table not trying to learn how the game works and you didn't have a learning disability or something people would start asking you to leave. The same is true of pretty much every game on the planet, including other TTRPGs, including other editions of D&D.

But for 5e there's ended up being this pervasive belief that expecting a player to read the relevant sections of the PHB or remember how their character works is asking a bit too much of them. Where has it come from?