Intel has become notoriously secretive with their documentation over the years, so I think that is very much the issue here. The 8086/8 were very open, but they started closing off little bits at a time after that. "Appendix H" in the P5 era was the first major sign of it. Bits and pieces have leaked out over time, but they still want to keep a lot of it secret.

For comparison, someone has made an entire 386 PC motherboard himself: https://news.ycombinator.com/item?id=29273829

I suspect there's enough information out there publicly to do everything up to perhaps a PIII (Socket 370 era) or thereabouts, but it starts being much harder beyond that.

A lot of existing (leaked) x86 mobo schematics are already available to anyone who cares to look. I don't think there's much "secret sauce" in a standard PC anyway, at least at the component interconnect level; it's almost like one of the worst-kept-secrets in the industry. There is a lot of commonality between all the designs. They might differ in the exact parts they use (e.g. VRM MOSFETs) but there's only so many ways to make something that works.