alloys suitable for casting are not known for their strength. It’s also very difficult/impossible to cast thin walled components.
so it’ll end up heavier than a similarly strong/stiff frame made from tubes.
motorbike frames are sometimes cast, when the manufacturer decides that the extra weight is acceptable, and when they’re sure that they can sell enough to bring the price down (casting small numbers is expensive).
and machining from solid takes ages. so it’s expensive.
because it’s expensive?
but they can of course do what they want…