It may be because a large enough load could quickly block the driver’s left-side mirror.

Oooh, another thing is that a heavy enough load may imbalance the vehicle enough to make it unsafe on turns.

Anyone see anything else?

https://www.thedrive.com/news/the-most-single-cab-truck-ever-was-built-to-haul-construction-materials

Then you could add forklift forks and you have a sideloader!