Because getting those perks by rolling your dick on the keyboard doesn't give anyone any sense of accomplishment. It's the same issue Blizz is having right now with the RNG TF and Legendary system. Killing Xavius in LFR and then having his loot proc to TF 910 doesn't give the person the same sense of accomplishment as spending countless hours on M Gul'Dan and finally downing him after like 100 attempts and getting a normal 900 ilvl loot from him.
Do you think the AFKer who just got carried through LFR Xavius in literally 5 minutes, because that's how long that entire queue takes should get the same AotC achieve, and The Dreamer title as the M raiders who spent tons of effort, coordination, flasks, repair bills, and time to down him? If you do then you should just quit WoW now and go play Rollercoaster Tycoon.