Many motherboards do allow splitting the GPU lanes into two it's just not many consumers see the need for an NVMe drive +GPU+ 2nd high speed device + 3rd high speed device that can't be served off the chipset instead.

For the prosumer crowd if you do something like the z790 you can have x4, x8, x8, and x8 to the chipset to be split as needed (x16 to be split if 3.0.