Precisely.

What would you want as a buyer. A driver that has already demonstrated that it is good enough to be included in the kernel, or one of unknown quality that may need extra work to integrate with the kernel.

I get why suppliers don't want to do the work. I just don't understand why there isn't enough value add for buyers to justify the premium for the benefits of a mainline driver, and/or why sellers don't try and capture that premium

I don't think buyers are actually going to pay enough for the sellers to justify the added cost. Remember that the buyers have to pass their costs on to their end customers (e.g. consumer phone purchasers), and those people won't accept all phones becoming $50 more expensive or whatever.

Also consider the cultural context. The culture of hardware manufacturers is much different than that of software vendors. They don't view software as a product, but more a necessary evil to make their hardware work with existing software infrastructure. They want to spend as little time on it as possible and then move onto the next thing.

I'm not endorsing this status quo, merely trying to explain it.

The way it seems to me is that a driver takes X hours to make, integrate, etc. It's cheaper for the vendor to spend those X hours, rather than each individual purchaser each spending those X hours.