Because we have at least 2 reports of it happening to UPS employees on this (currently very short) thread. Extrapolating the UPS rate of self-soiling to the entire economy, there should be literally thousands of people just pissing and shitting themselves at work every day.
But there aren't. Because most people work in jobs that let them take restroom breaks frequently enough that they don't need to soil themselves, and have access to a restroom during those breaks so that they can actually do so. UPS drivers don't: they may not have access to restroom facilities when they actually need to use them.
I doubt there will ever be an official metric, so it's your word against theirs.
I, for one, find the narrative that "they're just making it up so they can extort their employer for more money" to be less believable than than simply considering that it's the truth.
Furthermore, it was pointed out earlier in this thread that employees discuss engaging in this behavior _amongst themselves_. If it is, in fact, completely untrue, you would have to add "widespread conspiracy" to your acceptable narrative. (My reasoning being that there's far less incentive to lie to a co-worker than to a boss; only the boss can affect change. The notion that 260k low level employees have banded together to promote a lie, including amongst themselves, is so unlikely I'd call it laughable.)
According to employees, it happens frequently.
I extend my arguemets about whether or not the behavior occurred (at all) to the frequency of the behavior: either it's a lie or it's not.
Once again, seems to me like it's not.