There are of course also a lot of hobby projects and a few idealistic ones. I run some of those small projects myself. But most of the bigger strategic projects have companies behind them and are all about what they need and want. This long tail of smaller projects is nice and all and there's a lot of value there and people run them for all sorts of reasons. But as soon as they become valuable, companies and commercial interests tend to get involved one way or another. Most people just don't have a lot of unpaid time to donate.
Either way, decision making is strictly hierarchical typically. You are welcome to fork and modify and do your own thing. That's what it means to be open source. You are not welcome to force push your changes upstream. Getting your changes accepted requires scrutiny, consensus, and negotiation that involves the upstream people in charge. Mostly those people get that power by virtue of being there, doing most of the work, and being qualified for the job. And then getting paid to do that job. In most well run projects getting your changes in is a relatively straightforward process. But when there are conflicts, upstream generally wins.
Rails is a good example where loads of people started making money doing rails related work 20 years ago or so. The author is the creator of Ruby on Rails and the CTO of a company that drives its development and where the framework was initially developed. It probably makes nice amounts of money from doing rails related work and it apparently made him quite wealthy. Good for him.