Why doesn't castling reset the 50 moves rule? Capturing a piece or moving a pawn irreversibly changes the state of the game and thus resets the counter for the 50-move rule. Since castling irreversible changes the state of the game too, it seems to me that it should reset the move counter too, but it does not. Why is that?
I don't know the reason, but I can make a guess.

In standard algebraic notation, it's fairly easy to look at a scoresheet and see whether a capture has been made or a pawn moved.  Captures have an "x" in them, and pawn moves don't have a piece listed. But if loss of castling rights also triggered the 50 move rule, you'd have to look at the rest of the game (instead of just the last 50 moves) to see whether castling rights had previously been lost. This would seem to increase the burden on arbiters.

Also, it's rather unusual for this to matter. Most people are going to move their king (and/or both rooks) before they reach a point in the game where the 50 move rule would begin to apply.