Why teachers are biased

Guys and Gals ,
Fame gurukul Proved again that there are biased teachers everyhwere .I have met biased teachers from school to higschool to colleges in my life and here are some catagories I want to put them
1. Some teachers are bias to students who are always behind them and follow them blindly .

2.Some teachers are bias to students who try to butter them who praise them.

3.Some teachers are bias to rich students

4.Some teachers are bias to girls ( sorry gals even I would have bias to gals just kidding :))

5.Some lady teachers are bias to guys as well ( Even I had few :)
6.Some ( very few may be ) are not bias to anyone at all

Any thoughts ?

Peace 😊😊