That's because, like Hacker News, there are a relatively small amount of people who actually submit when compared to the number of people in the community as a whole. I personally subscribe to quite a lot of startup newsletters and follow people on Twitter who post product-related news, so I tend to have things to post most days. I assume it's the same for other people who also post on a regular basis.

People will use it for different things. Some will submit products, some will upvote, some will comment and get involved with the discussion, and some will do all three.

I don't know that this is true for Show HN posts (since it's a comparable product). It seems most Show HN's are posted by the creators. And Product Hunt is the other way around because most creators are not allowed to post there.