Being in full control of their own app, without conflict of interest from a competitor. Also they might use different programming languages, optimize for different environments they care about. And also to differentiate themselves from the competitor, to have an identity. Similar to how game consoles play the same games, but have their own identity behind it. And they want to sell their own subscription, so they only support and promote their own.

Just to name a few things that come to my mind. I don't use all those different tools, so I maybe wrong.