Why do people use IM slang?

One major function of slang is to identify the speaker with a group he aspires to belong to. Hence thieves use thieves cant, londoners use cockney riming slang, homosexuals use polari (or used to), and barristers and solicitors share a peculiar dialect they acquire while studying for their law degree.

Young people use IM slang because they want to identify themselves as happening and tech-savvy. If their parents started speaking 7337 they would rapidly abandon it (and invent a different patois).