You're not necessarily wrong, but mass gainers are cocktails of carbs protein creatine and shit loads of others. If you are working out properly you can definitely put on good muscle mass. On the other hand you can also over do it and walk around with a gut. Most people usually transition from a mass gainer to pure whey. And create their own mass gainer, I did that with whey, raw oats and blue berries. And every second, third day still do. Also some people find it hard to eat as much when they are that skinny to compete with how a mass gainer would help. Like everything else in life mileage varies.