Because anyone could make any job they wanted level 100, and still suck at the job. Wanting a job at lv 100 for whatever the benefit is doesn't prove anything other than that benefit is better than the other jobs' benefits at lv100. The only way to prove you know what you're doing, is to show people you can. And you get into these groups by showing you care about a job by gearing it properly. Any job can be geared well by players that know what they're doing.