You know, your point would be really well taken if all the cruise lines adhered to a formal dress code.
But they don't.
There are plenty of cruising opportunities where you don't need a tie or a fancy dress from one end of the cruise to the other.
With all those options available, why begrudge those who prefer something a little more formal just one single cruise line. . .Celebrity?
And yes, if people show up on formal nights without dressing, it does detract from the evening for those who enjoy it. And no, most of us don't let it ruin our meals, because we're intelligent grownups. But that doesn't mean we like it or think it's appropriate.