It increases engagement metrics, which is the most important thing for an ad company looking to raise more money in the future.
I imagine Reddit's website gets tons of traffic from "non users" coming in on shares and searches who will never consider installing a Reddit app no matter how many modals they put up but nevertheless driving ad impressions.
A product that is entirely based on user generated content is exactly the type of thing that requires active rather than passive users.
Mobile websites are generally optimized for passive consumption, and mobile apps – for active creation.