And because you don't know whether it is damaged or not (sometimes the damage may not be obvious), it's likely that the rocket will have to get a long post-flight inspection to check if it is suitable for another flight. It's something you can almost completely avoid if you land the rocket gently.

I read somewhere, that the costs of recovering SRBs of Space Shuttle from the ocean and then inspecting and fixing them were many times greater than building another pair of boosters.