I feel the comment well covers why GH stars feel more immportant; starring a repo is something you do specifically when you feel the repo really helps you out, you feel like you may need it in the future, or you think someone put a lot of effort into something and executed their idea well. That personal measurement, the number of people [developers] who think a repo is helpful specifically for their use, gives the repo validity for new developers looking for a project that can also be helpful to them. While this isn't 100% true for every GH repo, a lot of stars is a good indicator for usefulness.

Twitter and IG followers effectively do the same thing, but the issue is that it generally only measures things in the spectrum of "posts funny stuff", "is popular", etc. The GH star system has more weight because of what it represents compared to the counters of other social platforms.

I imagine professional Instagrammers and Tweeters(?) feel the same way about their RTs, likes and followers. But the vast majority of people on the platform are amateurs who don't care so much. GitHub has a far higher proportion of "professional" users in that context.