If I visit any SE site on my phone, I get the following banner on questions encouraging me to download the app:
It is possible to dismiss this banner, but you have to do this for every single site individually.
The problem with advertising the SE Android App is that it is far inferior to the mobile site I'm already looking at. Right now, it actually doesn't work at all, it just crashes on start. This problem exists for at least 11 hours right now, likely for every user of the Android app. A problem like this affecting the actual website would have been fixed in minutes.
The app is essentially unsupported now, it lacks features compared to the mobile site and is just overall a much inferior experience. There is a single reason to use the app at all, and that is push notifications. SE never invested enough resources into the app to make it a good alternative, and I can certainly understand the reason why they abandoned it now.
It doesn't make sense to advertise an inferior solution on the mobile site itself. The vast majority of users seeing that banner won't know that the app is unsupported and inferior to the mobile site. The reasonable expectation would be that the app provides a better experience than the mobile site, but it doesn't.