Of course that works but why isn't the protocol robust enough to do this automatically?

Because bittorrent is designed to be easy to implement to favour adoption, they could have created a more performant DHT and added something like eMule's automatic upload speed sense but that would have been more complex.

Why should it? I like that the protocol itself favours performance. Traffic shaping/QoS should be done by the maintainers of the pipes (ie routers or OS).