Holding a cat hostage by never letting it outside is a weird American thing.
Making odd claims that "indoor cats are happier" is also very strange. Not sure how people are measuring a cat's happiness.
Some cats don't give a flip about outside, some cats will not be contained. Treating an entire species with a one-size-fits-all solution is silly.
"Birds are much happier in cages, they should never be let outside, something might eat them".