There is no free lunch. Someone has to pay the bills on hosting, and I'm sure Reddit thought it'd be cheaper to do it themselves at scale than to pay someone as a middleman.
[0] https://www.theverge.com/2019/10/18/20920977/imgur-reddit-ns...
As I recall, it was Reddit who moved away from Imgur, not the reverse.
[0] https://old.reddit.com/r/changelog/comments/4kuk2j/reddit_ch...
[1] https://old.reddit.com/r/reddit.com/comments/7zlyd/my_gift_t...
The original UI works well, and it's great they kept old.reddit. Everything since has been horrible, along with the dark patterns pushing their mobile client. It reminds me of all the craigslist UI redesign posts on hacker news many years ago that completely missed the point.
Reddit has definite value (really important as Google quality declines IMO), and I'd love if they could find a way to sustain without destroying the community... but I'm not optimistic.