Actually most die from fatty liver or heart attacks because they are overfed. Snakes and reptiles in general dont need exercise like we do as humans or mammals. Reptiles are made to be able to conserve calories which is also why they can go months or even a year+ without food. They generally arent really active unless they are uncomfortable and trying to find a more comfortable spot or they are hungry and actively searching for food. A lot of snakes will lay in wait and ambush food.
I agree though that the biggest cage you can feasibly do is great but when you are talking an 18 foot female mainland retic, giving her a 36 foot long cage is not usually feasible lol.