That's not true, it will take you far longer to clean WOTLK raids than to clean BC raids, far longer to clean Cataclysm raids than to clean WOTLK raids, etc
I don't know many people that enjoy facerolling for 10 minutes to kill an outdated boss. For sure, it's easy, but it's also extremely boring, not so many people are willing to spend so much hours for transmog