who says they are ?
Hell who knows anything about them :-)

Presumably if you have sufficiently expensive scanners and a large scale printer you can make a cast that's custom designed to the limb in question.
And presumably it would be easier to remove and adjust than a traditional plaster or resin cast.

So on that basis I'd say a 3d printed cast would probably be lighter and more durable than a traditional cast and capable of removal to check progress and then the same cast could be easily reapplied, or adjusted to account for any changes.
So if the cast were being worn for a long time - then a removeable and reuseable cast would be significantly cheaper than applying a new cast each time.
It would also allow for more frequent checks at no extra cost.

On the downside even with a large nozzle, it'd take quite a few hours to print something as large as a leg cast - but if it were going to need to be worn for an extended period of time - then yep I can see a lot of advantages.