I should say it's not even specifically developers, it's just a broad assumption in the industry that people want things "fancy", that animations and loading and switching and so forth can take longer than .5 seconds because "people don't notice".
Every company developing apps only thinks about themselves and the OS they interact with, at most they'll include some share function, but OS-native theming? Multitasking performance?

The result is a slow piece of shit, that looks like a circus tent🎪