While raw cost is a factor, institutional inertia is also likely responsible for the longevity of tape-based backup schemes in many corporations where it might otherwise be replaced. Once systems, processes and people are in place to do the work this way, any change has costs and risks associated with it in all three areas, not just the systems.
The prospect of introducing new workflows and retraining staff is daunting to middle management in many institutions, so technology lives on beyond its cost-effective lifespan.