Well, what would you expect him to do?
Kotori’s route was more than enough to explain the relationship between Yoshino and Kotarou (and Kotori). Kotarou changes depending on the route, but Yoshino always remains Yoshino (except in Akane route where his character was destroyed and disgraced). Their ideals wouldn’t necessarily clash in every route.
Having Yoshino and Kotarou repeat that classroom scene in slight variation in every route would be pointless. Getting Yoshino too far involved into the supernatural on every damn route would feel unnatural.
I’ll say it again: Yoshino represents the normal life, the normal people. Yoshino’s short appearances in the routes reflect how the “normal life” goes on, what normal people do and what Kotarou might be doing now if he hadn’t gotten involved into the conflict. as well as the importance of that normal life aspect to Kotarou.
In Shizuru’s route, Yoshino gets a part-time job at Gen-san’s menra stall. Kotarou can freely talk to him. (They can talk about the good old days)
In Akane’s route (early middle), Kotarou ignores Yoshino when he tries to talk to him on the school grounds. (They’re worlds apart and Kotarou doesn’t care much)
Then again, his character does kinda feel wasted by not getting much screentime in the more serious parts of the routes. I really wouldn’t mind some other character taking on Yoshino’s role of representing the normal and having Yoshino get more involved into supernatural stuff and do stuff as you called it.