Because watching it on my computer isn't nearly as easy (stuttering, finding URLs, downloading players), convenient, relaxing, or high quality as sitting on my couch while watching an HD broadcast on my 50" LCD.
Easy, convenient: In the past I would have agreed with you. But by far, this year's olympics online broadcast has been very well managed. High quality with little to no stuttering (even for live events), content well organized and timely (replay of complete event available only minutes after they are complete).
As for the couch and large TV aspect, I suppose I would have to agree with you.
As for the couch and large TV aspect, I suppose I would have to agree with you.