I’m not talking about the technical rules of legal moves when your king is checked. I’m talking about when there’s checkmate and the victor and the loser are set in stone. Why can’t I capture the king at that point? I can understand why you can’t do so with a resignation because your pieces likely aren’t near the king.
you could originally capture the king in the predecessor sanskrit game but that would result in a lot pre-mature endings where the games felt unfinished especially since the king could escape the previous move. that’s why when it was brought into iran a rule was added so that you cannot capture the king, you have to warn your opponent that their king is in danger (kish or check) and until they get their king out of danger you cannot move. if regardless of whatever you did, your king would be captured the next turn then you and your opponent wouldn’t have any legal moves (kishmat or checkmate) and that’s game over.
so not breaking that convention doesn’t really need a reason since it’s basically a wasted and unnecessary move. it would be more noteworthy if you broke that convention since you are making a point about being defeated or fighting until your last breath or whatever.
also consider this was considered to be a diplomatic game, played between heads of states. if you just capture their king (basically killing them in game) even though it’s not necessary, that would send all kinds of wrong signals.
if there’s any other possible reason then i don’t know about it.
I like the rule because of its diplomatic implications. Basically it says “you’re in a shit position, but I’ll still let you negotiate your position.”