"I’ve never met a recruiter on there." I am by no means a fan/regular user of LinkedIn, but if you are an engineer in the Bay Area, LinkedIn is pretty damn great for getting random dm's from recruiters.
The ones who resort to spamming random strangers are self-announcing as the other kind.
But I did have one cold contact from a pretty legit recruiter that ended up turning into a pretty legit job. That may have been a fluke, but it might also be something that happens more often than I realize. Mostly I barely even read the recruiter messages; this one just happened to catch me on a rare day when I'm open to switching jobs.