Because Amazon also fires drivers if they don't manage an insane pace of deliveries-per-hour. Speeding is basically the only way you can meet the target they set. So it's pretty shitty of Amazon to put drivers in this Catch-22.
Going 45 in a 40 is a 12.5% increase on paper but maybe 3% more deliveries in a day at best. I'd be shocked if that much honestly.
Speeding does not help anyone.
With stack ranking, this can easily get you from the bottom half to the top half pf performers.
It's a large chunk of logistics. When EDL (electronic data logging) became mandatory a crapton of drivers left the industry because they could no longer fudge records.
The labor churn is an unintended, and ignored side effect.