[cfcef]

Arxiv is a voluntary service where authors upload their own papers. It covers a few hundred thousand papers, probably, since authors have to know about it, want to use it, and have copyright service; it covers only a few areas of science where Arxiv use is in vogue, and only from the past decade or two.

Whereas, SH/Libgen acquire copies of tens of millions of papers from everyone everywhere everywhen.

It's the difference between a local library and the Library of Congress.