The whole isekai thing

Am I the only one that thinks that was totally just thrown in their just so the main character would be like I know nothing but like that's fine and all but why couldn't it just been that he actually did loose his memory and was confused because he could remember everything the only thing that would change would be him knowing slangs and the jade but nothing big you know unless they do change something because him being actually 31 didn't change anything like he's not more mature or anything and he still sees the boy lead a 22 year old as a older brother maybe he couldve died at 19 or something you know