The framing in the question is wrong. Reddit gets ~80 comments per second on average. An API to get comments with ids starting after n would let you reasonably scrape all new comments with 1 request per second (or one every 10 seconds if the API returned up to 1000 results, which would be entirely reasonable). They get ~10 posts per second, so add another 1 request for that. The load is trivial. Storing the content is also trivial; the entire history of reddit fits very comfortably on a single consumer-tier SSD.

The infrastructure they provide is easily replicable at almost no cost (really it's just the bandwidth that costs any money at all). The community curation and moderation is done by volunteers. The content is all from the users. Reddit Inc is providing almost none of the value, and it's just benefiting from network effects. People go there because people go there.