Because, despite your outlying experience, cats generally show no emotional reaction to humans emoting, which is often (accurately) observed as being in contrast to dogs.
Also people generally mean that they have no empathy, not a lack of emotion entirely. Obviously you can scare or piss off a cat.
I'm not the one that downvoted you, but I will explain how to observe their emotions. Starting with the more obvious ones:
Purr noises: happy
Looks all puffed up: angry / scared
Puffed plus hissing: very angry / scared
Slow blink: trustful / nappy
Roll over showing tummy: trustful / give pets (not on tummy) / playful
Ears backward: listening / attentive / cautious / hesitant
Tail point tapping lightly: mildly irritated
Tail swishing: very irritated
Tail upright when walking: interested / happy
I've just always had cats, when a dog comes in, from my experience they just don't work out to well together. Switching to the Dog side.. cats can just stay over there.. get off my lawn! Don't poop in that sandbox!
Also people generally mean that they have no empathy, not a lack of emotion entirely. Obviously you can scare or piss off a cat.
Purr noises: happy
Looks all puffed up: angry / scared
Puffed plus hissing: very angry / scared
Slow blink: trustful / nappy
Roll over showing tummy: trustful / give pets (not on tummy) / playful
Ears backward: listening / attentive / cautious / hesitant
Tail point tapping lightly: mildly irritated
Tail swishing: very irritated
Tail upright when walking: interested / happy