It is an issue of expectations and affordances. If there's no clue that what's at the bottom is going to be different than what's at the top, and what you're seeing at the top is not giving you anything useful, why would you scroll further and waste your time seeing more of it? Perhaps for some people that is natural, but different people navigate unfamiliar spaces in different ways.

What you see when you open a github project page is a file directory. If you have no pre-existing reason to believe the page is something more than it appears to be, it's reasonable to believe that the page you're looking at is, in fact, just a file directory. There is nothing on the page which suggests that it also contains a readme file viewer, hidden underneath.

I didn't scroll because the directory structure suggested to me that one would navigate by digging around to see what was inside. After the first couple of times I tried this without getting anywhere, I wrote github off as a waste of time, and after that I simply closed such links immediately. It might have been easier to discover README.md from some later link with a smaller directory, where a smaller amount of scrolling down might have revealed it, but by then I was no longer looking.