It's certainly true that GL JS and Native never really got third-party contributors, and that the complexity of the task is a major reason; like you say, there aren't many GL hobby devs. Another reason is that they're simply great libraries and there hasn't been that much reason for anyone to contribute - I can only ever recall encountering one significant issue and it wasn't a showstopper.
But there's more to it. When the roadmap of an open-source project is tightly controlled by a sponsor, that can make third-party contributions hard or impossible. I know OSRM much better than MBGL so I'll cite an example from that - the distance matrix issue. This is massive for many users, plays right to OSRM's strengths, but Mapbox wouldn't accept patches to provide it for _years_. IIRC it only got in when Mapbox's attention shifted to Valhalla.
I'm not blaming the Mapbox devs at all for that; it wasn't important for Mapbox's business, and any extra code inevitably brings a maintenance burden. But it partly explains why third-party contributors are reluctant to contribute when the roadmap's out of their control.
Back on MBGL, there was/is a community, but the community formed around MVT rather than MBGL specifically - again, possibly because MBGL is just so good. https://github.com/mapbox/awesome-vector-tiles sums up the enormous community energy in the space (I think that might be your document originally, apologies if not). It's kind of a shame that MBGL being so far ahead of everything else has probably crimped development on alternative renderers.