I'm glad you asked that question, I've never understood it, never. It upsets my sense of balance and logic and I dislike those two bedfellows being upset. In a restaurant recently I ordered a naked tart (couldn't help it it was so cheeky), and that was delightful, the tart was housed in sliced vegetables and was ruddy lovely, but the waiter said nobody ordered it when they called it vegetable tart, now it's their most popular dish because it's called a naked tart, they also do a naked Manchester tart and something called a mooning copper (couldn't get to the bottom of that...boom boom
😉). So maybe the name is psychological and we all think omelette sounds boring but are more thrilled by the word crustless. Personally crustless conjures unpleasant images of scabs, but perhaps I'm peculiar in that regard. Free the frittata I say