Answers
Why do on-time deliveries get five stars automatically?
Because rating every delivery by hand is work nobody does. A delivery completed on time with no dispute is rated automatically; if something went wrong, the retailer files a dispute and overrides it.
In more detail
This is honest rather than flattering: an automatic five stars carries almost no information. If nearly every delivery is on time and undisputed, nearly every delivery is rated five, and a rating that never varies tells you nothing about which courier to prefer.
The approach is under review for that reason. A rating is only worth showing if it distinguishes one courier from another, which means rating what actually varies rather than confirming what usually happens.