Developers need a salary to pay the bills. Let's say that covers the first 40 hours of the week.
Those who are searching for significance outside their day job offer free labor as their "hobby". Maybe 10 hours a week?
For projects that want to move forward with some velocity it makes sense to make some of that development into paid day-jobs.
As projects get very large, there's a fair amount of overhead in just "keeping up". That erodes the 10 hours quickly. Further reducing the time to contribute.
So where is all this cash to pay employees coming from? Certainly not end users (as anyone who's tried funding an OSS project from users knows.) No, it comes from commercial companies (MS, Amazon et al) or venture capital.
This is the cognitive dissonance that underpins OSS development. The very people OSS treat as the "enemy" are the people funding OSS in the first place. As much as say RMS rails against big tech, Linux and the rich Linux economy system only exist at the level they do -because- of big tech.
Of course, I painting with a broad brush, and there are exceptions, but the point remains. It's turtles all the way down, and those turtles are not funded by users.