Well, I would say that your #1 reason is quite untrue. I can't speak for everyone, but I rarely browse Reddit on a computer. The only reason to do so is because I want to write a long response. Normally I'm browsing from an app so I can lay on the couch and watch TV while I'm doing it.
#2 could be true, but it might not be.
Regarding the apps, I paid for BaconReader on Android over 6 years ago and still use it. It works fine for me. I have no desire to switch to the official app.
As a moderator of a decent-sized subreddit, the number of people who complain they weren't aware of our rules because they use a mobile client (that makes it impossible to view the sidebar, where the rules are mentioned) suggests to me there's a ton of mobile users you're discounting.
Official app on iOS used to be a third party app called Alien Blue. Reddit basically purchased it, re-branded it as official, and viola. On iOS it had tons of usage prior to that. Incidentally after the rebranding is when I stopped using it and moved to Apollo.
1) Long sessions are more suitable on a laptop/desktop than on a smartphone (the average session time is ~15mn).
2) There is still a nerdy userbase (mostly early users) who prefer to use the website with all the features.
3) The app is relatively recent (April 2016 vs 2012 for Facebook).
4) The official app is not really good IMO (e.g. scrolling is laggy and the UI could be better).