I think there are other possible motivations, too. Not to say that you are wrong (you aren't), and not that this necessarily applies to Reddit specifically, but there is something to be said for having your service's icon implanted on the user's homescreen, possibly being one of the first things they see every time they open their phone. This opens up having muscle-memory to open up your app, making it that much easier to hop on and start doomscrolling. Having an app also makes it easier to drive engagement using notifications.
I think it's eye opening how unsuccessful web notifications have been on desktop. It's my belief that most people would not turn on notifications for most apps which send them if Android prompted like web notifications (I think iOS also defaults to allowing notifications without prompting for permission, but I'm not sure).
There isn't even an option on Android to block notifications except when explicitly allowed in settings. Each time you install a new app, you need to turn off its notifications.
I just think the entire concept of desktop-targeted notifications is superfluous and thereby it doesn't surprise me that it is unpopular. Like, if I want a notification about something I probably want it to go to my phone; sure, it is maybe nice to also be able to browse my phone notifications on my desktop, but I don't need two actual "notifications", as it isn't like I don't have my phone near me 24/7: what I'd really want is the "notification" to happen on my phone and then let me remotely browse a synchronized list of notifications (preferably end-to-end encrypted to the running phone, not using a user) from my desktop... but registering for a notification only on my desktop--or even my laptop!--seems like an extremely niche value proposition.
iOS does not allow notifications by default. Apps must display a system pop-up to request permission to display notifications, and the option is there to deny permission.
Whether users have already fallen into the same cycle as cookie consent pop-ups and, more recently, GDPR pop-ups, just clicking "allow" by default because it's the easier option, is a different question.
