I don't think they did it deliberately, but I'd bet someone said "We could increase engagement if people didn't have to click anything to keep seeing more content" and nobody bothered to think about other design consequences of automatically expanding the content downward forever.
EDIT: A screenshot of Facebook's in mid-2013 is shown here under the "Unreachable" header https://www.smashingmagazine.com/2013/05/infinite-scrolling-...
Or straight to the image: https://cloud.netlifyusercontent.com/assets/344dbf88-fdf9-42...
E.g. in Germany the impressum (often also called "imprint" in English versions of German websites; a mandatory page listing the physical address, contact information, registration numbers and other legal information, typically linked in the footer) must be easy to find and accessible with no more than two clicks (though this seems to be more of a heuristic than a strict legal definition).
They have useful info in the footer. But it's impossible to get to the footer.