Because their routes are optimized based on all packages. They will have to deal with customers who start questioning why the driver didn't take xyz path which

Because their routes are optimized based on all packages. They will have to deal with customers who start questioning why the driver didn't take xyz path which would have delivered his package faster (Though may result in slow delivery for everyone else). Perfect transparency is great in an ideal world. But we (Humans) are not all that matured :)