Arguably they buried it because it isn't actually what people want when they are looking for commit information. I know I've confused many developers by showing them gitk. I've been slowly working on a hypothesis that "subway diagrams" of commits look fantastic (Git Kraken looks great in screenshots) but get in the way of actually getting work done, much less thinking about the git DAG as an important tool beyond just "visualizing branches". (At this point I keep threatening to build a --first-parent based drill-down UI, that would look ugly as all get out in screenshots but might be a joy to actually use, but so far no one has sent me the check to make good on that threat.)
Yes, first-parent and expand would be a good way to view history. Github is a website and should be providing new and interesting ways to view information, not failing to provide even the basic lowest-common-denominator view (the gitk view).
You can't even get a simple gitk-style graph visualization on Github. Which is a website. After so many years, I really think all the blame for developer confusion can be laid at Github's feet.