Because humans are good at making assumptions.. And we need to generalize things to make better assumptions.
When you don't know about Hindus, and want to understand them, what they think, how they see the world/things, etc. Then you met a Hindu, you start observing them and also start creating an image about Hindu people. When you meet a Hindu next time, you recalls that image in your mind. And if that person sounds similar to the first one, the image gets more clear and strong in your brain, although your perception might be totally wrong and those two persons have different views than a typical true Hindu. But we are very good at making assumptions. Only few understands that everyone IS different.
