They do, but they're rare. I had my eye on a group buy for a left numpad mechanical keyboard, but it was like $300 minimum and it didn't even include switches or keycaps.

A right-handed friend of mine straight up switched mousing hands after I suggested it as a joke. I think he still mouses with his left hand to this day. I tried for a little while, but it was like learning to walk all over again and I couldn't get used to it.