I too have noticed this phenomenon. The apparent ubiquity of this practice gives me cause to wonder whether it might not be company policy to crapflood.

Crowded buffets cripple the efficiency of employees attempting to serve real customers. Additionally, you pay the same five dollars regardless of how much you eat. It's more economically effective for a restaurant to make 5 pizzas that will act as placeholders for an hour and then be discarded than to make 30 edible pizzas in the same span and in doing so risk pissing off real customers.

Another thing to note is how guilty the employees look when they're bringing out a pineapple anchovy pizza to the buffet. They look disgusted with themselves. They either walk with the solemn face of a remorseful executioner, or try to slither out, sneak the ringer on to the table, and scurry out of there, like a shameful ferret.