Well, unless you live in Africa, they're an invasive species. Cats hunt and attack wildlife regardless of whether or not they are hungry, which was great for humans looking to keep mice out of the granary but not so great for songbirds in the modern era. They do immense harm to bird populations in general, but even if you don't care about that, why would you let it:

Contract FIV
Get into fights with other animals or cats
Get hit by a car
Get ticks and other parasites
Eat poisonous plants or animals
Get wounded or eaten by predators

When on the other hand you could just keep it inside and make sure it has enrichment. If your cat really is miserable without the outdoors, catios are a thing. Cats are pets, not prisoners, and as animals that are not native to the majority of the places they end up in, they are dependent on their humans to keep them safe and out of nature's way.