Just to be clear, the two options in this false equivalence are "no fake USPS app has ever been seen, though it is possible in theory" vs "scam websites see as much traffic as USPS itself".
The first result in my region when I type usps into the google play store is a sponsored result for an app called Parcel Tracker, by internet media. The visible reviews for this app are negative citing scamminess.
If I search for whatsapp, in the sponsored section there are 10+ apps with white speech-bubble style icons on green backgrounds that aren't WhatsApp