Part of me suspects it is done this way for political reasons. Nobody wants a sewage reservoir near their house. Everyone is scared of a reservoir potentiall

Part of me suspects it is done this way for political reasons. Nobody wants a sewage reservoir near their house. Everyone is scared of a reservoir potentially bursting and flooding a whole district with sewage. Whereas if you say "it's a big pipe to take sewage away", nobody can object to that. Everyone wants their sewage taken away. The fact the pipe is also acting as a reservoir becomes a technical detail.