Why are people still using Scribd and Issuu? Where do their users come from and what alternatives do they use (or not)?

Scribd and Issuu were popular some ten years ago, when some browsers didn't show PDFs very well, but I thought they were made completely irrelevant with the emergence of default PDF viewers like PDF.js and Google Drive/Docs in the early 2010s. The more conscious/professional users can also upload their PDF very easily to archive.org or documentcloud.org.

Yet, I keep seeing otherwise respectable people upload PDFs and other documents to either Issuu or Scribd and force all their followers to run bloated JavaScript, see obnoxious ads and fight pointless restrictions to downloads. What am I missing?