In the wild, a cat that looks sick is a target, both to predators, and to rivals looking to move in on their territory. Evolution didn't select for cats that announce weakness, it selected for the opposite, and that instinct hasn't gone anywhere just because a cat now lives on a sofa.
The instinct that never switched off
A cat in genuine pain or discomfort will very often act completely normal in front of you, purring, eating, seeming fine, right up until the underlying issue is serious enough that hiding it is no longer possible. By that point, whatever it is has usually been building for a while.
Why this trips up even attentive owners
This isn't about not paying attention. Even people who watch closely can miss it, because there's genuinely very little to see. The signs that exist are subtle by design: a slightly higher resting breathing rate, a little more time spent alone, a touch less interest in play.
What actually works instead of watching harder
Since a cat won't reliably tell you something's wrong, the answer isn't more vigilance, it's measurement. A resting breathing rate, a weight, a note about behaviour, checked on a consistent schedule rather than only when something seems off, catches changes a cat is actively trying to hide.
Built around exactly this problem
This is the entire reason Halo's home screen surfaces changes automatically rather than waiting for you to notice something. A cat built to hide what's wrong needs a system that's watching anyway.