(first off, I apologize if this comes off as rude or argumentative. It’s not meant to, but I have a bit of a fever, so my brain is a tad mushy. The intended tone here is just “here is what I think and here are the reasons why I and a bunch of other people think that”.)
Definitely more of an American thing, but one that needs to spread. That post and many others explain why. Outdoor cats are much more likely to get sick or die violent deaths, and they don’t live as long.
Plus, they have massive environmental impacts. They kill rodents, yes, but also birds and reptiles. Any small prey animal nicked by a cat’s claws or teeth that manages to escape is at a serious risk of dying a slow, agonizing death of bacteria-induced sepsis from the bacteria in the cat’s saliva. They’re also responsible for the decline of numerous bird species, and have entirely wiped out several native marsupials in Australia.
Basically, it all boils down to this. The only benefits to letting your cat outdoors (aside from rodent control, which has other options) are that they’re less likely to get bored or overweight. Those things can also be accomplished indoors with proper care and toys, with zero risk of infection and no violent deaths of small animals. The downsides include serious risk to your cat and to wildlife that encounter your cat.