More benign versions of the same idea: app developers want to "provide a seamless experience"; they want their apps to be visually distinctive; they want to be

More benign versions of the same idea: app developers want to "provide a seamless experience"; they want their apps to be visually distinctive; they want to be portable across different OSes; they want clear boundaries between what they have to support and what they don't (is a copy-paste idiosyncracy something we have to document?); they can get performance advantages by implementing functionality themselves; and they're worried that the OS will change a critical subsystem or interaction in a way that isn't straightforward for them to adapt to.