Because, stupid, punches down, baddies, os biscuit, really real, shite, stupid, punches down. Oh did I sound like Sera? well, there you have her entire dialog for the game and, the reason I detest and, will never recruit her again. ( did it once, never again.)
Seriously?! It saddens me that people can't appreciate the points she is making because of her use of slang and non standard diction. Her dialogue is actually quite profound.
For instance, the dialogue where she uses the phrase "really real" is very interesting. She is pointing out that she, like many religious people in Thedas and the real world, consider religious stories morality tales without assuming that they are literarily true. But, as she points out, if Corypheus is to be believed, then there really is a literal Black City with a literal seat of the Maker. She, like any sane and thoughtful person, is frightened of the implications if all her religion's tales are literarily true.
As to her views on nobles vs peasants, not only is her philosophy sensible, it is far from extremist. She points out that she is part of a far flung informal network of non-nobles who work to fight back against the worst excesses of the nobility. The anonymity and informal hierarchy is clearly meant to protect members from reprisals. And her point about "Red Jenny", that nobles need that mythical figure to convince themselves that mere peasants can never pose a danger to them, is quite profound.
And if you listen to her banter with Solas, you'll see that she's far from extremist. She simply wants to address specific wrongs not overthrow the entire system, because she realizes that whatever replaces it is likely to be just as bad.