I think their opinion is that fries/salad is a lazy choice of sides to complete a meal. A traditional meal is usually a combination of protein, starch, and veg. In that regard, steak+fries+salad is a "meal", but it's definitely low-effort and designed to make people not hungry as cheaply as possible.

Good restaurants will create thoughtful sides that truly add to the dish, so it doesn't feel like you're eating "steak and some other shit".

I don't think it requires more skill, just more effort. Cut off a chunk of meat and add heat and you're most of the way to a good flavour, while veg dishes usually require a variety of vegetables, each of which requires more prep than the meat did. There are exceptions, but as a general rule of thumb if they're putting the time and effort into doing the vegetarian dishes properly it shows they're doing everything else properly too.

I recently watched a cooking competition show called America's Culinary Cup. One of the challenges was to make dishes exclusively using vegetables. It was nice to see