Last 2 jobs I was headhunted. But that's because I'm "old" in this industry (10+ yrs of experience) and have the right signals on my Linkedin = worked at good companies (Prezi, Facebook). The biggest signal is what company you worked at, second biggest what school you went to (if it's a big one).

I guess there's some segments where cover letters are a thing, eg. academia. But I've never seen anybody care about cover letters at tech companies.