Why a deep-dish slice needs support
A thin pizza slice can bend because its toppings sit in a relatively shallow layer on a broad, flexible base. Deep-dish is built differently: its tall rim and substantial crust contain a much deeper filling. Lift a slice by its edge, and the unsupported center has to bear the weight of that filling while the crust bends beneath it.
That creates a practical problem: the slice can sag, and loose cheese or toppings can shift as it moves. A knife divides the thick stack into manageable bites; a fork supports each bite from below instead of asking the crust to act like a handle.
The cutlery suits the way it is baked
Deep-dish is baked as a substantial, contained pizza, not as a thin round meant to fold or pick up casually. Its crust has to hold its shape around a generous interior, so the finished slice is closer in structure to a piece of savory pie than to a floppy wedge.
The knife-and-fork habit follows from that construction. It is less a rule about how pizza must be eaten than a solution to the mechanics of this particular slice: cut through its height, then carry the filling without making the crust do all the work.
