The issue with including transfers is that it's unclear if that player is already determined to be a "bust" or not.
Out of High School, the probability of a player becoming a bust is fairly well established for 4* and 5* players. If a player goes to Georgia, Ohio State or Bama, doesn't play much and transfers, that player me already be a "bust" at the college or P4 level, but gets counted in the college team talent index as if the probability is yet to be determined.
Examples are 4* transfers like Corey Collier, Stephen Wynn or Kaine Williams. All were busts at their previous programs and didn't play much at Nebraska. But their inclusion in previous Team Talent Lists made Nebraska appear "more talented" than the team actually was.