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Its because they have no free food in them and most people would need to eat 3 to fell vaguely full, and be hungry an hour later. Plus there's the stuff you put on top to make them taste nice. Do the maths

3 crumpets, 270 calories, jam on top (the lowest syn thing I canthink off) another 60 calories. For the same calories, you could have a 3 egg omlette, 2 grilled flat mushrooms & 2 grilled vine tomatoes.
SW is all about encouraging you to eat low calorie, filling foods, that keep you full.