I'll repeat my question, since you didn't answer it.
If the King can be captured, what stops the first player from using his Queen to capture the King and winning right away?
If the situation is not checkmate, it is in the player's interest to move the king out of check (or use another piece to protect it), otherwise it will be captured on the opponent's next move and the game will be lost.
If the opponent can capture the King if it remains in check on the next turn, what stops the first player from moving his Queen to capture the King on his first turn?
I'll repeat my question, since you didn't answer it.
If the opponent can capture the King if it remains in check on the next turn, what stops the first player from moving his Queen to capture the King on his first turn?