Because its boring? ! WE did gear just gear for many expansions, it's boring as hell after a while, I much prefer these other systems, wow is much more than just logging to go to your raid and pick up ypur tier set, thnx bye.
While it is there to keep you subscribed, it als o keeps playing regualrly rewarding and more interesting. It's a pain when you don't have time to do as much as you want though and can't be bothered to have a new grind every patch because you already did that alot. But how else will they keep you subscribed and or interested?
You may be ocontent with just normal vanilla gear, but many people get bored fast, and it becomes stale fast. Besides if itwasn't for al lthis, the nostalgia and enjoyment of classic wouldn't be so pronounced. You have to have moved away from something for a long time for people to have fond memories and want to have a go at the old system.
Calssic provides that now, but it would be disastrous for live to go back to that, and boring as hell. Live's average age is also younger than classics. And there is a reason new players are a lot more on live than classic. you can't just cater to the past, gotta look to the future.