This is part of the price that is paid with open source - if you open source it, it is understood that people copy/improve what is there and it moves the field ahead, but you don't get to directly monetize your code. At the same time, the dc42 fork of RRF is deeply, deeply dependent on previous people who have open sourced their work, making it easier to have built the dc42 fork of RRF in the first place. And this branch of the firmware has been forked to support Smoothieboards, and I doubt that any of the manufacturers of Smoothie, SBase or SGen boards are sending any money back to pay for further RRF development.
The monetization model of open source is that you give away the code, and hope to make money on the support. So it makes a lot of sense to get a payment system in place for people who want to donate. Klipper has a Patreon donation setup, it would make sense for RRF to do the same