Why are chicken thighs sold as bone-in skin-on or boneless skinless but never boneless skin-on?
Actually while they are not easily found everywhere, some stores do sell the boneless skin-on chicken thighs and breasts. In fact, I just bought some of the thighs this past week. (I'm in the US, mid-atlantic region.)

The reason most stores don't sell them that way is because it's a less popular option and therefore in less demand than bone-in,skin-on or boneless/skinless. That said, if you shop in stores that have butchers on premise, you can ask them to bone the thighs and leave the skin on.

Back many years ago before boneless/skinless pieces became popular, it was quite easy to find the boneless thighs and breasts with the skin on.