Cause cameras, among other things, militate against the "simplicity" of the game. For some, it's important that the game played at the highest level be as close as possible an approximation to that played at much lower levels. 99.9% of all games of football are played in pretty basic conditions, and it's not like goal line cameras could ever be used in these circumstances. I suppose the thinking with using the extra officials is that it's at least reproducible on the African Savannah or some such. And personally, I quite enjoyed seeing Chelsea having a perfectly good goal disallowed yesterday cause the officials couldn't keep up with play.