It can be more of a fame game, like YouTube video creators who started doing it for free, or musicians. The more relevant it is, the more people access/use/play it, the greater the chance that the name of the proponents would be known/recorded/reported. It can serve as credentials for future paid work.
It can be a way of giving back to people, knowing that most of us stand on the shoulders of past workers and thinkers, and also that God has given us more than we have ever worked for. Opensource programmers, maintainers, support providers, team members, reviewers, bugfixers contribute tools and solutions to the community both for today and tomorrow. They strive to plant good seeds, and see if these take root and bear good fruits.
Opensource stuff promotes teamwork. Some do what they can. Others use what the first group has produced to form more complex things that the component producers couldn't make on their own, which benefits both parties. The work of opensource software creators could come full circle and help the thinker-implementor, their family, kids and grandchildren and immediate neighborhood.
Opensource software (which can be extended to hardware or processes) enables for more collaboration. People would be able to help improve it or correct its flaws. It would be possible for others to continue to keep it working, when the original thinker-implementor(s) are no longer able to do so. The effort of the thinker-implementors are maximized and continues on long after their energy and motivation are gone. With it we are more able to keep creeping obsolescence at bay. With it we are able to create more robust, mature, sustainable, modular systems.
Whether they receive monetary income or not for their work, it must have occurred to most, if not all, opensource thinker-designer-implementors that there are so many elements of existence that are more important than money. And that money is just a tool, a tool which in its use is today is also improved by opensource software. Even without receiving money, opensource people receive so much more. It is indeed blessed to give. And there are people who would like to support these opensource work, individual people and big companies who have integrated the opensource stuff into their operations. Thinking of ways to make it convenient for people to contribute any amount at their chosen time and frequency is a good idea that would help encourage the thinkers-designers-implementors.