While having a very long, complete answer is interesting, there's a very simple tl;dr that answers the question just as well. You only have N bandwidth availab

While having a very long, complete answer is interesting, there's a very simple tl;dr that answers the question just as well. You only have N bandwidth available to the house. Cable companies have a tradition in providing content; content can be very complex, and very rarely was content ever coming back from the house. So, instead of doing N/2 upload N/2 download, they did, lets say, N/4 upload 3N/4 download. Why haven't they fixed it? Legacy systems. We've all been there.