From my experience developing for both iOS and Android, it's because iOS users expect your app to look good, or they won't use it. Apple just sets a higher bar

From my experience developing for both iOS and Android, it's because iOS users expect your app to look good, or they won't use it. Apple just sets a higher bar, both because of Apple's own apps and the approval process, and developers code to the users' expectations. Plus, with the nice looking built-in UI widgets and Interface Builder, it actually takes some effort to make an app look bad.