If it sounds like I'm complaining, it's not because I'm frustrated, but because it's slightly offensive to see that being demanded. In my experience, their internal recruiters monitoring inbounds are usually ringing me before my finger is even off the send button. They don't seem to look for or care about a cover letter, and I haven't had trouble getting offers, even from top tier companies. And honestly, I don't even think I'm that impressive. From what I can tell, anyone who has fogged a mirror at a half-respectable tech company already will at least get a phone screen these days.
When I was previously involved in hiring, I don't recall ever seeing cover letters on inbounds either. When asked in person by potential candidates, I always told them not to.