It's very possible we've not had success breeding them in captivity.

There's a number of species that we've either struggled to, or outright haven't been able to breed.

Squids, for example, were previously thought to be impossible to breed in captivity because they are very difficult to keep alive and healthy in standard aquariums. Once we figured out that we needed HUGE tanks with opaque walls in order to keep them alive, we began to see success in breeding.