Because at my work, clients want the general public to be able to install their apps sometimes, and my department does native apps, a different department does web stuff. If you want Joe Public to be able to install your app, you have to go through the Google or Apple stores.

However, we don't directly dev in Kotlin or Swift, we use Unity, which can export directly to an APK, or an XCode project (which has to be compiled a second time which is a nightmare). Sometimes we have to write native plugins to access device specific features (Last time I needed to was to access the FoV of the device's main camera), but it's not common, Unity has great support for most things you want to do out of the box

EDIT: I forgot, there are some situations where we supply an android or ios app to clients through our own app store, but I believe that devices have to be "enrolled", especially for ios. Generally though, Joe Public will want apps through the official stores