Presumably because that's been our experience with human societies? You might protest Germany in the 1930s was "modern" technologically but still "barbaric" morally - but I think that overlooks the way all the European powers were behaving in Africa and Asia, barbarism wasn't so uncommon as all that. And it's less common now, even as we've made more technical advancements? I don't think we develop on all axes in lockstep, but there's still a general trend, and I'd be pretty surprised to find aliens advanced enough to come visit who didn't already have plenty of resources, so why bother messing with us?