1 inspector/100 facilities seems within the realms of reason if you're doing spot checks. It's absurd if you think you need to be doing continuous audits.
It sounds like you're saying there's good reason to think the slaughterhouse business needs the latter (that there is a documented history of abuse despite them happening - I don't really know anything about the industry), but most industries seem to survive on spot checks.
I don't know much about the zoos and breeders that the USDS is inspecting either, maybe continuous auditing would be a good idea, but it doesn't seem absurd to think that spot checks are sufficient. Is there some particular reason to think that they're like slaughter houses and that spot checks are clearly insufficient?
To me, industries that are responsible for the well being of animals are more similar to old persons homes, hospitals and kindergardens - they are responsible for the well being of individuals that can't care for themselves. Being at the mercy of the workers, the individuals need better protection than is afforded by spot inspections.
Another thing that should raise scrutiny in these context is that animal welfare costs money, and companies are there to maximise profits - which creates a perpetual tension between respecting animals rights and better/faster results/profits.
And a quote : Freedom is the right of all sentient beings - Optimus Prime
For residential, building inspectors certainly don't work systematically, at least in the US.
When and what they check is pretty spotty.
My understanding of building inspections in Toronto, Canada was that they were supposed to be reasonable systematic and thorough even for houses, but I'm definitely not a reliable source. Even if my impression was right I definitely have no reason to assume it applies elsewhere.
Anyways, hopefully the point came across anyways even if the example was sub-optimal.
If an inspector checked something and if they noted the flaw then it would have been addressed. Or they could have just not checked, not noted, or missed it.
Same with home inspectors, who are under no legal liability except for the most gratuitous and obvious flaws.
Which is contrary to how most people assume the system works when they're buying a new-build house.
Some practices might not get done in those days - and might not be reviewed. Imagine no animals has to be put down during these two days, there will be no actual review of that procedure - and so on.
In slaighterhouses in Canada, we have federal inspectors during all work hours but activists still film abuse happening.