True, but those flaws and weaknesses can be enhanced by a varied array and many (not all) will want that. Taking 18's across the board is fine, but not everyone will want to.
It's like a coin flip; often you already know the result you want and the result you get can help to acknowledge that e.g. If the coin shows X and I'm not happy with that result I know I should take Y; just taking Y in the first place removes the need to flip the coin. In this case (choosing your scores) you remove the need to roll dice or impose arbitrary limitations from point-buy or standard array to build the character you want.