Game dev is a different beast and in most platforms the development/tools are the same or close enough, also when building a game you rely only in low level API (Vulkan, Metal, OpenGL, etc) from the main platform you are targeting. But when you are building an app then you use the hight level API, that means you don't need to deal with font rendering, etc. You could obviously build an app with Unity, but it will not feel native, also IIRC Unity is awful when you need to input text in your game (in iOS at least, not sure how's Android side), it appears a input element above your keyboard while when using the native UI the UX is different.