It’s the same reason as the third-party app API changes: prevailing forces within the company want to know that for every X API requests made, Y targeted ads were put in front of eyeballs. Most mobile browsers can block ads at this point. Plus, native apps offer more ways of gathering user data opaquely, which is worth lots of money in the eyes of finance people even if they haven’t started doing it, or if they expect it to be limited in various ways by mobile OSes and/or laws. The profits vs losses just point them this way, at least according to certain financially-minded people. This isn’t really a new phenomenon.

If Reddit execs thought they could take the same approach on desktop (forcing us all into desktop apps with unblockable ads and more system access), they absolutely would. They see that Slack and Discord did accomplish this effectively, and probably want to catch up.