Unix is(was?) a commercial product. Linux was born out of the desire for a free version of Unix. There are economic incentives.
I agree that money isn't the best motivation for people. However, in general, things like robotics, medicine, silicon fabrication, etc. are out of most people's leagues financially. In order to be able to innovate in these fields, you need to convince some company that they can make more money than what they will spend by researching in the field.
Remember, patents cover more fields than just software.
Unix was originally a non-commercial product, albeit produced by paid researchers at Bell Labs (owned by a commercial entity, AT&T). And actually, Wikipedia claims that {Thompson, Ritchie, et al} received no financial support for the first version, which was developed on a spare PDP-7, so I'm not even clear if they were using paid time to work on it. It was non-commercial because AT&T had been forbidden by a 1956 consent decree to be commercially active in the fledgling computer space. It was only in 1983 (with the breakup of AT&T and the consequent expiry of the consent decree) that commercialization of Unix started, over 15 years after its creation.
So the successes of Unix pre-1983 should be lumped with the successes of GNU (founded that same year) and Linux, as being non-commercial and not-for-profit.
I agree that money isn't the best motivation for people. However, in general, things like robotics, medicine, silicon fabrication, etc. are out of most people's leagues financially. In order to be able to innovate in these fields, you need to convince some company that they can make more money than what they will spend by researching in the field.
Remember, patents cover more fields than just software.