I think the main reasons are that it gives people with nothing insightful to contribute something they can share and can elicit some emotional response which is easier for a picture than text. A picture is also faster to consume and process which likely explains the popularity of image macros and memes. Another problem is that the majority of posts in such subreddits are usually the same few questions, so typically a sticky post or a wiki page will be made to refer people there (or asking for help will be specifically banned) and suddenly there is not much to talk about.

The most egregious example for me is r/selfhosted which started as a tech DIY subreddit and transformed into people posting pictures of all the docker containers they are running at home.

edit: In fact, I would go as far as to suggest that the primary reason HN is slow to succumb to the Eternal September is because it doesn't allow for embedding media.