[Tenoke]

It's a good question as most games, including chess have few rules about making otherwise legal moves illegal just because they might be suboptimal. In novice games the other player might not even realize they can win, so it being illegal to stay in check is an oddity but one that's actually in the rules.

[bspammer]

As far as I’m aware, the only game-theory difference between chess and a variant of chess where moving into check is legal, is that stalemate exists. Otherwise the “can’t move into check” rule is basically useless.