Also, safety. There is a walk bike path near me and I prefer biking than stationary bikes, but not everyone has access to safe paths. Even the relatively safe path I use, I have maybe permanently injured my right wrist due to a fall caused by a dog jumping at me (he was just an excited sweetheart, but that didn’t stop me flying off the bike). My sister also walks the path and has been chased by feral dogs, plus has to deal with creeps hitting on her. Lots of people walk with some sort of blunt instrument for safety, though personally I feel that’s over the top.

While the only serious danger and injuries came from animals, that’s still something you wouldn’t have to worry about in a gym. The creeps… maybe, but still safer in most gyms than out in the open.