It's because these people fetishize liberal democracy as the pinnacle of political systems. This what the infamous "end of history" from Fukuyama is all about. The idealization of the system leads them to rationalize its contradictions rather than honestly confronting them.
Libs will claim to care about human rights and dignity, but they always ignore structural issues like wealth concentration. Their cognitive dissonance stems from a fear that the mere acknowledgement of the flaws delegitimizes the entire system. As Wendy Brown put it, liberal democracy's moral claims often mask its complicity in perpetuating neoliberal exploitation. When push comes to shove, they will always side with capitalist imperatives.