I'm speaking from the perspective of the IP owner who writes the driver and manufacturer who puts together all the components.
As am I.
And I'm sure the drivers would get mainlined.
That's not the norm.
Intel
Intel is huge and employs shit loads of Linux developers. Most vendors, who will be much smaller, don't. For example, Realtek, who stick a crappily written driver in a tarball on their download page and call it a day. Or any of the hundreds of silicon vendors (such as NXP, Nvidia, Rockchip, Allwinner, Realtek again, Qualcomm, etc., etc.) with "BSP"s who give their customers a 500GB package containing, among lots of proprietary userland shit, some butchered horror show based on Linux 3.3 with no git history.
I can't imagine why you would expect drivers to be mainlined by a vendor.