Ultra intense competition. The companies do what they need to do to stay alive and satisfy their shareholders. Fedex and Amazon are fierce competitors for UPS.

Amazon in particular has incredible power since they can decide what they themselves will ship, and what to send to others to ship. This allows them to self-ship lightweight, regular sized packages, which are the most profitable things to ship, and send the harder to ship items to carriers. It is a brilliant form of arbitrage.

On top of that all of the major parcel companies are at least partially competing with a giant, government subsidized entity (the post office).

We are coming off of a long recession, meaning the workers themselves have been grateful just to have job. Long recessions breed labor abuse, because the workforce is willing to accept it.

Lots of reasons. I am sure it is technically unlawful to make an employee shit him or herself, but what is allowed is not the driving factor here.