People who say this make me laugh because it is extremely obvious that you don't exercise, either indoors or outdoors. It's like asking "why would anyone ever buy an oven when you literally could just eat the raw ingredients", presumably you've just never consumed food before.
There are benefits to stationary cardio. 1) More convenient 2) Easier to zone out 3) Built in pacer 4) Safer 5) Doesn't matter if it's snowing/raining/humid/hot/dark. 6) Can be done from home, relevant if you have kids, on call duties, etc 7) Good way for adults to learn how to ride a bike in the first place 8) two people can run next to eachother at different paces while chatting 9) can be stopped at any point and you're never 2 miles from your destination 10) Some people just don't enjoy riding in the road, and riding on sidewalks is illegal in many areas 11) can be set for very long downhill or uphill segments that aren't realistic in the real world 12) easier to follow a class/routine.
Like. People aren't just idiots. They use them because there are good reasons to use them.