Because they thoroughly fail to understand the world, their own position in upholding much of what is fucked up and awful about it, generally refuse to educate themselves about how and why that is because it means the difficult recognition of their own complicity, and then are often super arrogant toward basically anyone, including folks who know a whole lot better than they do, about almost anything, politically.

So that's the charitable take, for the libs who are basically kinda decent internally but just super ignorant and unwilling to learn or sacrifice. The ones who consider themselves already "on the right side of history", and who think that is the hard part - having already done it, they are ready to answer questions from others and be congratulated!