It's also a bit more legitimate because those radical atheists usually come from within the society they're criticizing. Not always in the sense of having been devoted christians themselves, but just being white and born in the US means you're more of the "in-group" of a white christian than others.
I'd also like to mention that you're operating from a stereotype of "average leftists" that I feel is overly broad. What you see on tumblr (or what you see when someone shares a screenshot that looks like tumblr) is far away from the average. Regarding uber, check this 500-comment threat to see that there are many people on HN critical of their business practices, especially in regards of drivers' rights and wellbeing: https://news.ycombinator.com/item?id=9731963