Well, the gang doesn't actually seem to do much updating -- especially Shaggy and Scooby. You would think that after the first dozen monsters turned out to be people in costumes, they might start reducing p(need to run away) upon encountering a "monster", but this never happened, as far as I can tell.
Think of the individual members of the Scooby gang not as separate people, but as separate modules in the brain. Our inner Shaggy and Scooby never stop freaking out, no matter how convinced we are that there is no actual monster. But we do stop taking them seriously and trust our inner Velmas.
A great column by Chris Sims at the Comics Alliance.
Excerpt:
Tim Minchin fans may recall him mentioning Scooby Doo in a similar light in his beat poem Storm, and it's been brought up on Less Wrong before.
When viewed in this light, Scooby Doo really is like an elementary version of Methods of Rationality.