Because they stink.
(I crack myself up.)
But, great question. Honestly, it feels great. I own “nice” socks, but during the summer and warmer weather it just feels good to go sockless. Maybe it’s not for everyone, but I think it’s just more or less about that breeze across your ankles.
Or maybe it’s just a perk of not having to rummage around your sock drawer in the morning for a matching pair.