Why do we use hearing instead of using hear in phrase "l am looking forward to hearing from you"?
As far as i know after "to" people must use infinitive.

Question

Answer

When you look forward to you look forward to something. The gerund is replacing a noun.

Answer

The infinitive includes the word "to" - to cook, to drive, to be.

I want to cook dinner now.
He knows how to drive.

"to" is also used as a preposition.
I go to the shopping mall.

Your rule that "to" is followed by a infinitive is not a good rule.  Where did you learn this rule?

Answer

"Looking forward to" is an idiomatic phrasal verb and "hearing from you" is the direct object of the phrasal verb.  'To' is a preposition particle and is not looking for an object of a preposition.  It is part of the verb phrase 'looking forward to ' and not referring to an infinitive.