Why isn't the handle and bottom part sold together?
This is the way the manufacturer sells them. The bottom brackets are used for another purpose for boats, not just for this application.
I only purchased the product, but I believe it is sold seperately because the support brackets can fit into a track that can be mounted to the side rail of a boat, to hold the tanks while on the water. If you want to use it as a carry aid, then you can purchase the handles. That is my best guess.
I wondered the same thing...I have never been able to use the handles because I never bought the base! Suppose I should do that one day.