How does PLOS ONE maintain its impact factor?
There are, I think, two distinct factors at work that may help explain some of your puzzlement:


Your field's impact factor is not academia's impact factor. For example, society journals in my field have an impact factor of ~ 5, and some of the big names for very splashy studies have impact factors ranging from 20 to 56. Depending on the balance of fields submitting to PLOS ONE, their impact factor may be coming from more cited fields.
Long-tailed citation papers. Impact factors, like many averages, are susceptible to long-tail effects. PLOS ONE is an open-access journal, and a highly visible one. It's possible that the occasional highly accessible generalist paper makes it there, and yields a large number of citations as a result, pulling up the overall impact factor.


Both of these are helped, in my opinion, by the lack of review for "importance" - beyond your suggestion that this does result in less important papers ending up in  PLOS ONE, it's also a benefit to papers that don't quite "fit" in highly specialized journals, but may still be impactful.