I can corroborate most of this. Basically it's a stressful job, with low pay, weird hours, and very high turnover. Some people handle the stress better than others. Generally when people leave, it's because they burn out, or reach some kind of "last straw" moment, or get physically injured (often the back, from lifting improperly and/or too fast). The company doesn't care, they're just using you up and they'll hire a new one when you leave.

"Fragile" and other special instructions printed on the box are kind of insulting if you think about it. On the one hand, I'm a "professional" doing this for a living, so it's not like I need instruction from your I've-never-loaded-a-truck ass. And on the other hand, do I look like I have time to give your precious package special attention and follow your additional instructions for how it should be handled? It will be handled like all the rest. Maybe worse now that you pissed me off.

My pet peeve was the packing. If you've done a proper job of packing, i.e. fulfilling your responsibility to secure your stuff, you quickly find you don't need to proclaim "Fragile" on the outside. But if you don't pack well, and then write "Fragile" on it, you're kind of trying to transfer your responsibilities to me, and deserve a lesson.

This was judgmental young me talking, by the way. It's been a while since I did this. But speaking of judgment I also should mention the ultimate worthlessness of most of what people went to the trouble to ship. I handled a lot of B2B stuff, such as a region's worth of cigarette advertising for all the (insert name of convenience store chain here) in that region. Invariably marked "Fragile."