Why do many people have prejudice and over-generalisation in how they perceive others? Is that some sort of defensive mechanism or laziness or what?

If Jesus chose to save mankind with prejudice, then many of us are excluded. But no. Jesus had no prejudice whatsoever. He saves whoever comes and repents, no matter what.

I want to be like Jesus and to extend his grace without prejudice.

Related:
Prejudice trumps intelligence
Empathy