Because you don't have much choice, and each choice has trade offs. If you pick the part from vendor A, you get the mainlined driver, but maybe you get slower performance, or higher power consumption, or a larger component footprint that doesn't work with your form factor.
And most vendors are like vendor B because they're leading the pack in terms of performance, power consumption, and die size (among other things) and have the market power to avoid having to do everything their customers want them to do.
Still, some headway has been made: Google and Samsung have been gradually getting some manufacturers (mainly Qualcomm) to support their chips for longer. It's been a slow process, though.
As for mainlining: it's a long, difficult process, and the vendor-B types just don't care, and mostly don't need to care.
And most vendors are like vendor B because they're leading the pack in terms of performance, power consumption, and die size (among other things) and have the market power to avoid having to do everything their customers want them to do.
Still, some headway has been made: Google and Samsung have been gradually getting some manufacturers (mainly Qualcomm) to support their chips for longer. It's been a slow process, though.
As for mainlining: it's a long, difficult process, and the vendor-B types just don't care, and mostly don't need to care.