> why does anything needs to be open source in the first place?

Not advocating FOSS approach for anything, and I'm not strictly speaking of GH in that context, but generally, malicious code loves being hidden into closed source software since it's the best effective way to keep it hard to control and correct. When privacy, security, safety, accountability become important, the FOSS approach might solve a problems or two. Of course the downside is that everyone can look at it, so building a billions worth business out of a fork + 10 line changes + renaming of a FOSS project cannot work without violating the license by keeping it closed.

Not sure about others but I probably learn way more from reading sources of open source projects than I do from blog posts and books.

There are so many issues with GitHub that could be easily addressed if open sourced. So many in fact that there are entire unofficial GitHub repos dedicated to raising issues about GitHub. They won’t see any benefit now that they are owned by MS and slowly being integrated to AzureDevOps, but they could definitely have had a much improved platform if they had opened years ago.

It's a bit ignorant to claim that public knowledge access has no value. There's a lot to learn from reading #1 project of it's kind, don't you think?