Let's face it, once you cut into a pie it's just a mess. It's much easier to maintain the aesthetics of a pie if each person has their own.
This taps right into that toddler part of the brain where you want to pick up a whole pie and smash your face into it. Taking a big ol' spoon and jamming it into a jar of cake and frosting and having no regard for the mess you're inevitably going to make sounds so childishly satisfying. Where's my big spoon?