Because for me, and I suspect many others, there's a great deal more to be considered than just the dress code for a few hours each evening. For example, itinerary, embarkation port, price, solo supplement, and the ship itself.
As long as someone is prepared to adhere to the dress code for the venues where it is required, why should they miss out on the main aspects of a cruise simply because they don't wish to 'dress up' for 'celebration nights'?