Because they take one sentence from the vet's verbatim of the vestige of the information from the textbook/experience and only focus on that. Imagine the vet says about five paragraphs worth of information about the effects of obesity on cats, and has one line about "his blood panel doesn't show anything outside of the normal range right now." The owners focus on just what confirms their view.

[–] bane_killgrind 3 points 1 day ago

"he's fine for now" truncated to "he's fine" and then HE DIES AT 7 YEARS OLD