I remember reading about the NGDV, and one of the reasons it looks so weird is because USPS wanted a vehicle that was low to the ground (to make it easier to climb in and out of) and easy to see over the hood, even for very short drivers[1]. Given that they are in residential areas (and thus, in proximity of kids playing outside) far more often than UPS/Fedex, I can't say I disagree with that requirement. (Also, if you have a tall truck like UPS and Fedex typically roll, good luck delivering to the average mailbox while staying in your seat.)
USPS has certainly evaluated more traditional designs; in fact, they are actively using ~20k Ram ProMasters (a rebadged Fiat Ducato), which are quite similar to the Mercedes Sprinter, alongside ~9k mini vans[2].
[0]: https://www.usps.com/business/every-door-direct-mail.htm
[1]: https://x.com/Nir_Kahn/status/1364465483911675905
[2]: https://www.uspsoig.gov/sites/default/files/reports/2023-01/... (PDF page 6)