User blog comment:B214/Question and Answers 2/@comment-187.209.0.247-20140226163658/@comment-1359322-20140227001116

Even if Issei becomes a High-Class Devil now, it would still take him few hundred years before he could start searching for his own team you know. He has lots of factors to consider, the funds he would need when he becomes independent, the amount of money he would have to use to take care of his servants and probably a territory of his own, which is something he can only attain by being an Ultimate-Class Devil.