They can. It's just that mealworms are one of the worst (nutritionally) feeders. People get a bit over-zealous when it comes to feeding them to their animals. They are fine, if appropriately sized, and not a main part of their diet. Everything is usually fine in moderation. Really, a diet with mealworms is probably better than one without - variety, nutritional variety.
When they're very small, however, veileds often dislike really crunchy things - so make sure they're not big at all.
My little guys, about 2-3" snout to vent, are eating medium sized superworms now. They coudl easily down a full grown mealworm.