The problem is not that they don't enjoy it enough, the problem is that, for the majority of 25 man rosters, they spend two times as much time recruiting and managing than they do raiding! Which means, if that particular 25 man guild is running 10 hours a week, it means they are recruiting/managing up to 20 hours a week.
It's not that they don't enjoy the 25 man raiding scene, when they are raiding. It's that the massive timesink of recruiting and management is such a royal pain in the ass that it becomes less desirable to deal with without incentives. Incentives obviously make it easier to do both jobs.