Fiberglass is great, and I too do not know why f/g cages are not seen more. Being that modern cage plastic material is relatively lightweight and definitely machinable, or in some cases moldable, I bet that is why it is used almost exclusively compared to fiberglass over wood or other material.

I saw a monitor cage years ago. It was a rough wood shell that was covered in fiberglass on the inside. No way a lizard even with claws was going to destroy that. They even built out a structure that looked like a bench, and they coated that too with f/g to make a step/shelf in the cage for the lizard to climb up on to bask. I say go for it.