I do not think that your 'indoctrination' theory is either the most obvious, or correct.
My (equally simple, imo) explanation is that critically thinking about the world and the people in it is both the main purpose of academia and far more of a left wing ideal.
The "those who support my political opponents don't just have a different perspective/ideology, they are also uncritical and stupid" has seldom been useful - or accurate. It's basically self-congratulatory.
My (equally simple, imo) explanation is that critically thinking about the world and the people in it is both the main purpose of academia and far more of a left wing ideal.