that’s a lot of tooling to build for a single application—plus not everyone is tech savvy and installing from non-standard locations requires more user support

Not every device is under MDM in a big corp. Often you have people like external consultants bringing their own devices, who need to participate in (semi-)internal communications. You cannot just MDM those and you cannot just issue bigcorp devices to them, so you need something like the normal appstore to distribute the software. Maybe you even have BYOD for internal people, so MDM could be hairy from a GDPR/employee rights/liability standpoint. And maybe you even have customers and partners who you want to communicate with, whom you have to provide with a viable option of communicating. You can (maybe) separate those into an internal and an external communication tool. But then you just have two different tools, one of which will have the exact same problem about needing installation via commonly available appstores.

It's easier to simply ensure that the apps you need are present in multiple stores.