When you will be ranked to member or at least Junior member, you can try to enrol into sigmature campaign that you like, and then they might accept you
Now a days the most important thing for getting accepted in a campaign is the post quality... you might have a sr. member account but if your post quality is poor then it would be hard for you to get accepted in a good campaign.
Your point is right, and i saw the profile of the person who has wrote this thread, actually i think he never want to do his work properly, even now in this time he is Sr. member, but now he is not interesting in it. May be he don't know about to join the signature campaign and also he think it is not suitable for him.
Have you checked his latest posts and activities? Maybe he is not active in the forum anymore because i don't feel someone with a sr. member account would not be interested in signature campaigns, or maybe you are right that he feels he cannot do any constructive posts that is why he is staying away from it.