V6's assembly version also does less. The reason GNU's is so complex is because it has a line-numbering feature (cat -n) not supported in V6 or V7, and also tries to read and write large chunks, to avoid overhead from calling stdio functions in a loop. It also tries to take advantage of non-portable extensions where possible, but fall back to portable code when not supported. Yeah, it looks a bit complex at first, but it's not really that bad if you actually take the time to read it.