My pal Buster, a dog, goes outside every day. But I would never dream of allowing a cat to go outside.
When Buster goes outside he is accompanied by a human. For his safety he is kept on a leash whenever he is not playing fetch in a secure area.
I would never, ever consider opening the front door for Buster to spend the afternoon outside unattended. I doubt he would make it 10 minutes on the streets of San Francisco without suffering serious trauma.
But unattended outdoor activity is standard operating procedure for many cats.
If dogs were allowed outside in the way that many cats are, veterinary emergency hospitals would be as common as gas stations or convenience stores. Canine life expectancy would drop 80%.
I know many cats who are allowed outdoors only on leash (cat harnesses and leashes are readily available at pet stores). None of these cats is likely to get into fights or be hit by cars. They are, however, at risk of contracting feline leukemia.
Cats and dogs are not equipped with the wherewithal to thrive outdoors in human society. It is our responsibility to look out for them.