doubt about a checkmate

I am a beginner. Some time ago I faced a position similar to this. I thought that if the queen captured my pawn I could simply capture the queen since the black rook can't move. However it is mate. I've been thinking a lot about this position. What do you think? Should the king be able to capture a piece defended by a pinned piece?

A king can never move into a square that is covered by an opponent's pieces and pinned pieces still attack squares. If you were allowed to move into an attacked square, exposing your king to check, so could your opponent to capture your king, if that was allowed.

The rule is clear and explained in post #2. If you want to change the rule you need to provide a better concept. Undoubtedly, long ago, kings were captured rather than checkmated - until someone decided that was too barbaric for a noble game. By the barbaric standard black in your example is still the side that captures the opposing king first and therefore deserves to win. Look at the finish of the I robot movie. After the central computer VIKI was disabled all of its NS-5 bots stopped fighting on the spot. Once a chess king is eliminated all of its army is immobilized as in this movie. No more opportunity to capture the opponent king!