I don’t know if companies have invested millions in making products. I would believe that they have made millions in selling them however. The only exception it seems is the water based extender, that seems more thoughtful than most. Then again, I don’t think I have seen the whole range of products out there. In fact has anyone made a list of all products available?
I like the one I have made, I can do stretches in a ‘c’ or ‘u’ or ‘s’, or up or down, also complete twists. the only problems are consistently getting a strong pull, and it digs in at the base and head.