It seems to be a choice motherboard manufacturers make during the design phase (where they wire in which interface will share with which other interface), as there are boards that support what you seek.
Unless I'm misreading the manual and specifications, The
GIGABYTE GA-X99-UD5 WIFI for example runs x8/x8/x16/x8 when all four x16 slots are filled (and x16/x16/x8 with three cards). The slot that runs permanently in x16 mode doesn't share with any other interface or slot.
But in all honesty I'd prefer them all x8, with some of the remaining lanes wired in to an M.2 Ultra slot.