There are comparatively a lot less than github but they do exist.

Some examples of successful project using gitlab.com: Wireshark, QEMU, Inkscape, F-Droid, Aurora, Commento, Kicad, Remmina, tortoise-git...

Maybe sadly true, the social network effect. But through distribution (packages etc.), that can be very untrue, and anyways people who really want to meaningfully contribute will find their ways (and probably understand the benefits of free/open-software).

Hopefully, Veilid can help solve the “on its own little island” effect of a decentralized platform.