Because half-upgraded, half-unupgraded AI's have horrendous chances of rewards. D:
Actually, here's what happened. Once upon a time, way back in the day, there was only AI0, 1, 2, 3, and 4 (T50). FG's were slowly introduced with Quests - more FG's were added over time. After a while, level 5 was the last one to be added - it was supposed to be a mid-level enemy between AI3 and FG's.