Because that makes the basket empty quicker. Towels are easy—rectangular and regular and their corners touch and the creases cooperate. Then t-shirts, but you have the check the label to know if it’s your dad’s or your brother’s. Men can be ambiguously similarly sized.
Always leave socks until the end. You can hang them over the basket and locate their twins more easily that way. Your lazy brother (he of the ambiguously sized t-shirts) may even walk by and claim his socks with a flick and another flick.
If you fold fast enough the piles retain their clean fresh smell and you can bury your nose in them for a treat at the end.