Which ones are you thinking of? Does Grab operate this way?
The China case is well-known, but that's really its own beast. KakaoTalk (Korea), while more of an 'everything' app than those in the West, is still a far cry from containing a mini app store. A user can't choose to add any new functionality by installing something - it's all included right from the get-go. My (limited) experience with Line (Japan, Taiwan, Thailand) is similar. So I'm curious if there's any non-Chinese apps you can name.
FWIW I'm not arguing against your fundamental premise, would just like to know which do-everything apps you mean.
Though if it's similar to Line I wouldn't even count Zalo either, as said, from what I've seen with Line it doesn't have an app-store-in-app, all functionalities are predetermined and preinstalled. It just does a lot of things, but that in itself doesn't go against any App Store rules.