Why can't you endanger your king to a pinned piece?

If you can grasp it, then surely you see why it makes sense within the rules of the game.

I understand that it is illegal to put your king in check, but if a piece is pinned then your king technically isn't in danger, and when that piece is unpinned it will just give a check. Why is this against chess's rules?”

I’m going to try to rephrase what other people have said to try to make it more digestible, but first let me point out that you are operating on the assumption of perfectly played chess. For a moment, consider what the impacts would be of your interpretation of this rule for less adept chess players.

Think about this scenario under your interpretation of the rule:

Let’s say you put the enemy king in check across the 3rd rank with a rook and the opponent moves a pawn to be adjacent to the king, blocking your rook. Your king is capable of moving to one of the squares the enemy pawn is attacking (where it defends one of your opponent’s rook, perhaps), so it does, capturing the enemy rook. As you’ve noted, the logical thing to do would be for the enemy king to move, initiating check through their pawn, but, again, this is assuming that every player would make the logical move. Say the opponent *doesn’t* move their king and opts to attack your pinning rook along its current file. Your interpretation would make it illegal to capture the attacking piece with your rook because disengaging the pin necessarily results in you delivering check to yourself. In other words, you’ve pinned your own rook along the 3rd rank with your own offensive maneuver, which is counterproductive.

In short, if you initiate a pin and put your king in a position where he would normally be checked by the pinned piece, you forfeit your ability to disengage the pin until you move your king out of check, meaning the opponent could essentially force you to either commit to a bad capture or go without full functionality of your piece for the remainder of the game.

Of course, there is some nuance to this, and pinning your own piece doesn’t necessarily guarantee you are going to lose (you could still try to find a way to checkmate along the rank, in this example, with a queen per se), but your interpretation of the rule adds unnecessary complexity when a simple “The king cannot be moved to a square where it would be placed in check,” avoids the problem I’ve presented and reserves limitations to the king as opposed to making every Bishop, Rook, and Queen capable of pinning themselves outside of situations where they are defensive blockers.

TL;DR: Chess is complex, easy to learn and difficult to master. Your interpretation adds a degree of weird that makes the game more difficult to learn. Attacking the king with a piece shouldn’t result in your own piece being pinned, and your interpretation encourages that.

Mad respect for having the courage to question the rules. Hopefully my post helped you understand one reason why the rule could be interpreted the way it is and not the way you’d like it to be accepted. Cheers