Because implementing designing/manufacturing/validating SR-IOV HW is expensive. It's not something that would be useful as a differentiating feature for most consumer grade HW.
There's lots of hardware competition for consumers, including upcoming Arm laptops from Mediatek and Nvidia. Intel can use feature-limited SKUs in both CPUs and GPUs to target specific markets with cheaper hardware and reduced functionality.
The silver lining seems to be that more and more things are moving into the firmware, and it's entirely possible that SRIOV could be supported through that in the future. But till then, I doubt it's going to happen.