Because grown up Dipper is the awesome Dipper. did I mention I was their age when the show started. meaning years wise he'd probably be sixteen by now (Despite the fact that the show ended a few months ago and they were Gravity Falls
We're all upset about the ending of our show. It brought us all together. Hundreds of thousands of people have a picture of dipper on their wall, or a Mabel sweater in their closet, maybe a Fez on their desk, or a journal sitting on a nightstand.