But ... hardware manufacturers might just try to outsource driver development for cost cutting reasons.
Well, I'm sure they are using the cheapest engineers that they can find, given the required need for performance, quality, timely availability, and so on. Given the complexity and the requirements, they are probably spending a fortune on very good software engineers. Why: Like all manufacturers, NVidia sells a system, not a piece of hardware. Part of the system is firmware and drivers. They have to make sure that the drivers and firmware are very good, or else their stuff won't sell. That's best done by throwing money at the problem.
Those wanting to sell hardware should also deliver tested Open-Source-drivers for the BSD-world.
Not sure that it makes financial sense. For desktop usage, Windows has a market share of ~80-90%, Mac OS (which is irrelevant to aftermarket graphics cards) has about 10%, and all others are in the single digits. Of that, Linux is the lion's share, easily 10x more than the BSDs. Then you also have to remember that the BSDs are fragmented into three branches, each with smaller market share. It might just not be worth it for NVidia (or any other graphics card vendor). Unless they can find a way to work mostly from the same source code, with a software architecture that isolates platform dependencies (which can be hard).
Another thing to remember: A significant fraction of NVidia's revenue and profits don't come from desktop usage of graphics cards, but from server usage (where they are called GPUs). That is a market in which Linux dominates, with Windows second, and the BSDs once again very far behind.
I'm just a user so I don't know the details of how hardware makers actually make money,
NVidia makes a TON of money. Just look at their beautiful and gigantic new buildings on San Tomas Expressway. I see both the finished building and the construction site roughly once a week (when I do some shopping on the way home), and it's BIG and beautiful
... but I think open source would reduce overhead for makers since driver development can become more of a community endeavor.
As shkhln already said, that is an area where volunteers and amateurs are unlikely to make much headway. The amount of work needed for a full-function high-performance GPU driver is probably dozens or hundreds of man-years; not something a college student will put together in the evening in his dorm room.
Remember, a significant fraction of Linux today is written by paid professionals, who get their paychecks from companies like RedHat, SUSE, the various Linux foundations (which are funded by industry giants), Google, Facebook, Amazon, and IBM / HP / Oracle.