My tuppence'th - you want to put money in the ring when you have the best cards. When you have AK or AQ there's a good chance you have the best cards (or at least cards that can improve to the best hand). That's why you raise.

Remember, your opponents are also unlikely to improve on the flop - don't just check-fold every time you miss - use position, reads and image to win some pots that by rights might not be yours.