Yet almost all apps that I use are open source. OsmAnd, Firefox, Telegram, RedditIsFun, Linux Deploy, AFWall+, AdAway, Barcode Scanner, ConnectBot, Quasseldroid, some games like Lichess and Anuto TD, and a bunch of other tools that I use less frequently. Closed source is what I paid for, so fairly trustworthy as well (Spotify; Es File Explorer and Smart Audiobook Player). There are of course a few remaining ones like Google Play Store (I have F-Droid next to it) and Google Maps for traffic (use it 2-3x a month maybe).

Why don't more people use open source apps? I don't know, it's kind of weird since Android is a much bigger audience than, say, a Linux desktop. (On my laptop I don't think I use a single closed source thing anymore, at least not regularly.) I guess Android has a similar market environment as Windows instead of the open source ecosystem that surrounds Linux desktops.

I suppose it's like asking why websites are almost never open source. Why indeed?