Socialist Leveling

I've been thinking a bit about the commonly-heard complaint that socialism, once implemented, would impoverish the vast majority of the population. Leveling, it seems, is always viewed by critics as something that would act to drag down the vast majority of people. Yet, if one considers real mixed economies in western Europe, they have higher standards of living and lower poverty rates. While there are obvious differences - Britain, for example, is far more stratified a society than Denmark; France has social problems related to immigration from former African colonies and the reaction of the population to them that don't exist in Norway - and the relative smallness and homogeneity of the populations might make a difference, I'm still curious as to how this notion began. Actual socialist countries tend to level up. Why do critic believe that won't, or can't, happen here?