To start with, Angelica is the protagonist of a tragic story.
Her relationship with her dad is pretty superficial, and her mom was too busy to show her any real love. To deal with her loneliness, Angelica dreamed up the Rugrats and their adventures.
But why would a toddler need to dream up a bunch of babies to hang out with?
Because at one time they weren’t imaginary.
Chucky and his mom died in a car wreck, which is why his dad is such a neurotic mess.