Usually collaboration is done using a Git-like external version control tool or just file syncing with Dropbox. Even Adobe Photoshop stores your data in files that you can share as you like. Figma's collaborative approach is certainly good UX, but on the whole a net-negative for the world if it catches on.
If your code is stored exclusively on GitHub and can only be edited through the UI then you are at GitHub's continued mercy to have access to your own data. You lose it when your subscription expires or when GitHub thinks they no longer like you. This is the general trend of software turning into services and they are actually locking you in with your own data. But it is also a challenge to provide seamless realtime collaboration on the same document if sits in files rather than on the web. The best of both worlds would be a software that has both the convenience and openness of individual files and has seamless collaboration.