I think it's because they are usually targeting the design for the broadest audience, so they design for the lowest common denominators and simplify the screen

I think it's because they are usually targeting the design for the broadest audience, so they design for the lowest common denominators and simplify the screen as much as possible to streamline "the main flow" of their app. This involves removing as much elements as possible to cause the least confusion when they interact with the app for the first time. This means that the app will work amazing and without thought ("it just works!") for 80% of the population but will be non-functional for the 20% of the population which includes power users and users who would have been willing to go through the initial learning curve for a more powerful app. Just to note: I'm not deriding the "lowest common denominators" as I am definitely one for some domains. For example, I chose the app with the simplest deisgn for doing my taxes as I didn't want to spend time learning about taxes.