And biking to exercise is a different form of exercise than riding an exercise bike. Raising resistance is not the same as going uphill. Maintaining your bike in balance and steering/controlling it is not the same as sitting on a static frame.
Not to mention, the overall experience is completely different, and that can also matters for reasons related to quality of exercise.
You can't bike for transit "at home".
You can't bike for enjoyment "at home".
And biking to exercise is a different form of exercise than riding an exercise bike. Raising resistance is not the same as going uphill. Maintaining your bike in balance and steering/controlling it is not the same as sitting on a static frame. Not to mention, the overall experience is completely different, and that can also matters for reasons related to quality of exercise.