Why do most teachers not try to save bad students?

In middle school, there was a teacher who tried to save a bad student, he made
the good students and the worst students sit together, in order to
encourage the struggling students. The best-performing girl
ended up sitting with the worst troublemaker.
As a result, seven years later, the teacher was called by a friend to a KTV, and when
he was looking for someone to sing with, he found this girl,
the teacher then talked to her, asking why she had come to this point.
The girl didn’t speak, and after the teacher left the KTV, she reported him,
saying the teacher had harassed her, and it had started since middle school.
The teacher said, I didn’t do anything, but everyone didn’t believe him,
and his reputation was completely ruined. His wife also divorced him.
The teacher was unwilling to accept it, unable to understand why the girl
accused him falsely. She then asked the teacher in return:
Then why did you make me sit with the troublemaker?