Microsoft's standardization got AMD and Intel to write upstream Linux GPU drivers? Microsoft got Intel to maintain upstream xHCI Linux drivers? Microsoft got people to maintain upstream Linux drivers for touchpads, display controllers, keyboards, etc?

I doubt this. Microsoft played a role in standardizing UEFI/ACPI/PCI which allows for a standardized boot process and runtime discovery, letting you have one system image which can discover everything it needs during and after boot. In the non-server ARM world, we need devicetree and u-boot boot scripts in lieu of those standards. But this does not explain why we need vendor kernels.