I think, there's one more reason, why in all the regulations, the lowered mast position is required. It's Murphy's Law + the HSF (human stupidity factor).

The working hydraulics on many trucks is still operated mechanically/hydraulically.
It means, that even when the truck is switched off and disconnected, you may lower the mast just pushing the lift lever.

Murphy's Law says, that for sure, one day, someone (probably unwillingly) will rest his arm on the lift lever while somebody else is just doozing below the carriage (HSF)....