That hasn't been my experience. I usually cite printed sources in articles when I write them, and most articles I edit that were written by others seem to also be based primarily on books.
Some statistics would be interesting, though. I would imagine online sources are more common in articles on recent events, and in articles on popular culture. In the area I mainly edit in (pre-20th-century history) it's almost entirely books and journal articles.
A good citation to a printed published work grants you demigod-like powers when editing an article - if the work itself is notable. If it's self-published or has no information demonstrating it is a reliable source, not so much.
Yeah, I've found it really reduces any hassles over "notability" if you're citing solid published sources. If I'm writing an article on a Greek archaeological site, and my article cites The Oxford History of Greece and the Hellenistic World, case is pretty much closed.
Some statistics would be interesting, though. I would imagine online sources are more common in articles on recent events, and in articles on popular culture. In the area I mainly edit in (pre-20th-century history) it's almost entirely books and journal articles.