Dude. Have you ever had a dog? Every had a dog harness? The designs are very similar.
Cat harnesses can absolutely be made so that the cat doesn't slip out. My cat's harness is designed so that not only can he not slip out, it will also not Choke him if he gets hooked on something.
For my cats we were lucky enough to live somewhere that had very little traffic at the time, so we'd let the cats out only when we were out.
Then slowly let them out, but only in the garden area, and then only with a leash. Eventually we stopped letting them out. We'd distract them before opening doors.
Grow up.