A pizza is a static physical object that, if divided too much, becomes so small that it loses its "function" and can't be shared among people.

Bitcoin, on the other hand, can grow in value and, if divided, the smaller units can still be as useful as the bigger ones. We could reach a point where we Bitcoin keeps growing in value and we keep dividing, creating a virtual infinite supply. I say virtual because the max number of BTC will always be the same, but the division of it creates valuable units that retain their original function.