LATE EDIT*** Imagine if character creation was in for a second. Imagine spending some time to get a character made (might seem complex to a D&D noob) and having him die in one of the encounters. The D&D noob might be put off at the time it took to create a character to only have it die in a short while later. Pregens give them a taste for the system without the prep or attachment of a self-made character.
You just described exactly how all my friends got hooked on Blue Box Basic, and made them want to play more, and do better. For 30 years.