I don't understand why osmo 2 is not on the DJI Go 4 app? Why need to separate?

My best guess it that is was too expensive to merge them all into the DJI Go4 app.
It was re designed with the latest experience they had.
DJI Go already had support for the Osmo, so it was cheapest to use that for the Osmo Mobile (2) as well.

My guess it is because it would need effort and investment to migrate to another platform.

Thanks for your suggestions and feedback. And please understand that osmo series are in the same app.

Installing two huge app in one phone it causing a lot of space to the phone itself, DJI Go is not small app.