Again, because his base personality is passive for default. If you read Maruto previous works -Saekano, White Album 2-, or Shikimori-san, the direct inspiration of Maruto for this work, the message is about ROLE REVERSAL
All Maruto male MC started being submissive characters and ended being submissive characters. Their growth is in the opposite direction than do you want.
The series message is about is fine a boy would be submissive and passive if he gets being in a relationship with a girl disposed to takes the lead
He was passive because Hikari didn´t love him in middle school, period. And of course, Hikari unawarely sending him to the brotherzone worsened his inferiority complexes.
And he doesn´t flee in this situation. He wrote to Hikari, tried to talk with her, and she is avoiding and ghosting him. All this after PUNCHING him in the gymnasium. So, other mysandric poster with zero media literacy, other mysandric post supporting violence against men.