Because they're creating "make-work" jobs for otherwise-unemployable drones. Thedefinitelyly could eliminate one day of delivery a week, though I'd prefer to keep Saturday delivery and skip Monday instead. Special Monday deliveries could still be made, at "postage-and-a-half", presumably with a small skeleton crew on overtime.
“Thedefinitelyly” = “They definitely”.
Spell Checker gone astray. :-)
I worked there a quarter a century ago and I wish they hadn't made the work so hard. If it was just "make work" they should have made it a lot easier than bending, carrying and sorting parcels from midnight to nine AM. This otherwise unemployable drone would have appreciated a soft day job in a dust-free air conditioned office.