I don't consider side projects a good metric either. I hardly have side project for a simple reasons because, I get pretty much burnt out by the daily work itself.
Unfortunately salary hoppers are so common among the job hopping crowd, that its sufficient enough to say the most and nearly all job hoppers are salary hoppers.
If you won't change your filters, you will continue to get your usual stuff and hope against hope that you get someone who will be "loyal." Ain't gonna happen and you need to maintain a team of "HR loyalty maintainers" who fill your inboxes with peppy mails about morale ...
I just told you about what works for some of us. Sure it might cost us a couple of good people who don't have a side project, but all of the people we end up getting are the problem solving type.
Unfortunately salary hoppers are so common among the job hopping crowd, that its sufficient enough to say the most and nearly all job hoppers are salary hoppers.