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.