Such a silly question... which could potentially have thousands of different answers as not everybody raids for the exact same reason. I recently quit, but for me personally it comes down to have fun with a group of people. Back in ICC times I was in this guild and we weren't that good but we had so much fun as a 25man raiding guild.
Your point about gearing up being a waste of time isnt completely valid in my opinion, at least not in older raid times were vp gear and rep gear were unheard of. You need a certain ilvl gear to be able to clear new bosses from new raid instances, so therefore previous gear obtained isnt really a waste of time. Also doing something new beats killing farm bosses all the time.
The reason I quit is because 10man guilds (on eu-kazzak) are disbanding faster then you can say disband.. I had to find a new guild every 6 weeks or so, I just didnt want to bother with it anymore But evern I quit I didnt feel like I wasted all that time for nothing, I met a dozen of great people/players and I've alot the most fun online with those people.