Spam.
The target site may be spammy: lots of adverts, popup/unders, taboola "chumboxes" etc.
Or, if a site can make money from inbound link traffic, there's an incentive for its owners to spam links in subreddits to drive traffic to themselves.
> This is especially bad on apps where you need to know how to find the share option separately on each app
Definitely another problem, walled gardens that you can't link into as well as out of.