I mentioned this in a comment to another answer, but it seems important enough to make it a proper answer.
A hand like A, 2, 3, 4, 6 is an ace high flush. A hand like K, Q, J, 9, 8 is a king high flush. Make any card in either hand of a different suit and you lose the flush but you still have Ace high and King high. Ace high beats King high every time. The value of the remaining cards is irrelevant unless you have a tie. This is why A, A, A, 2, 2 beats K, K, K, x, x. Three of a kind is harder to get than a pair so that is why that part of a full house gets priority. The remaining pair is used only as a kicker if the three of a kind part is a tie.
It's quite possible to create a new game where you get rid of kickers and replace them with some kind of averaging, but that game wouldn't be poker any more. It would be a more complicated game requiring a lot more mental effort. You might say that anybody can add five numbers, but poker is already best as a fast paced game, where pros think in terms of number of hands per hour where larger is better due to the fact that more hands means return being closer to the long term average in terms of edge producing a profit. Having to add five numbers repeatedly in any calculation you do for pot odds, comparing your hand against what your opponent might have, etc. is going to really slow the game down.