Why do weight conscious WRC rally cars use steel roll cages?
The simple answer to that is because the FIA tells them to. Before the rules were tightened up they got upto all sorts of shenanigans.
You could build a carbon roll cage, plenty of series allow them. But that would require far more testing and cost a fortune. It’s easier to say roll cages must be made from a specified grade of steel, set OD and wall thickness and say they must be triangulated a certain way. Then all your scrutineer has to do is poke his head in, check the manufacturers plate, and tick a box.
Comparable to the UCI weight limits. They could come up with a durability test, but it’s simpler to just say bikes have to weigh 16lb and remove the incentive for manufacturers to get into an arms race.