Very good question, the rounded rectangles are lesh harsh than ordinary rectangles. Psychologists have also stated that rounded corners are more soothing and welcoming. Hence why allot of apps and UI designers use rounded corners.
Rounded corners are more visually appealing than just ordinary rectangles as well, at least in my opinion.
I've read an article 2 days ago, take a look at the difference:
I personally know what you mean by how apps have similar designs/concepts. Which is why I'm going to try to be more creative and innovative, as well considering the User Experience for users. As a UI designer, you need to make sure that it isn't difficult for users to navigate when using interfaces and inputting information.