Why do anything then, as all you do will be eventually replaced, right? Then your life will end, and all that 'progress' will be for nothing. Edit: it isn't just about you, others matter as well.
Now, actually being to the point, the reason why people raid and do these types of things is probably for the personal challenge. I mean, why would people do the hard modes of a game? Personal challenge, and overcoming that challenge, refining your self to be able to complete that, testing one's abilities to the maximum, knowing one's limit, surpassing that limit, attaining one's potential, gain knowledge; however way one wants to call it. These are just personal reasons why (I) some people may want to do hard mode or difficult things in games, including raiding or heroic raiding. On an individual level. And just simply to have fun.
Then there is the multiplayer aspect, which generally comes down to raiding with friends and enjoying the moments you play with your friends, whether they suck at the game or are good at the game. Helping one another surpass their own respective limits, and overcoming obstacles together, as friends. And just simply having fun together.
Gear isn't the sole motivator for raiding; physical rewards are not the sole motivator for doing difficult things in and out of games. People do it because they want to, or they have personal motivations for doing it, not just for rewards.