Come to think of it, we come across actual magical radio broadcasts at several points. Even magical password-protection of same. Why the hell do they still use owls?
Because delivery of packages to rural areas is terribly unprofitable.
Certain things need to be socialized in order to keep a country connected and enable development.
Without things like the post office, rural electrification, and the Eisenhower Interstate system, vast swatches of the US would have zero economic output.
Context is important.
It's a fair question though - anecdotally, the two main uses for the postal service nowadays (in countries with internet access) are package deliveries and unsolicited marketing (possibly print subscriptions factor in).
To my mind, the unique advantage of the Muggle postal service for personal communications, apart from the globally federated physical distribution network - the one that would make me very hesitant to scrap it - is the difficulty of conducting mass surveillance on it.