GenZedong

It's part of the cycle of blame. Liberals can't take responsibility for failing cause that'd mean they have to actually do something more than whine about what the rightists are doing; they would have to both obtain power and leverage the organized violence of that power in order to suppress any and all rightist influence. But they don't and in practice, often ally with and enable them instead. That leaves them blaming the nebulous, shifting, ill-defined entity "the left". An entity which is portrayed as both weak and strong; on the one hand, "the left" is viewed by liberals as an inconsequential sample of the population and thus something that should be ignored when it comes time to legislate or court votes. On the other hand, "the left" is viewed by liberals as a serious threat that undermines their ability to win elections by refusing to support them and carrying water for what they label rightist talking points, such as (at this point, with the vote blue no matter who nonsense) criticizing anything a liberal in charge is doing.

If they took responsibility, they would have to admit that the liberal order they idealize is impossible to manage or sustain or implement meaningfully much at all without the contradictions building over time rather than lessening. They would have to admit that where they stand isn't in any kind of middle, but is in direct opposition to the marginalized, the colonized, the working class. And the best they can ever do as liberals is the political worldview equivalent of Scrooge voluntarily becoming a good guy in A Christmas Carol. They can't take it further than charity unless they take power seriously and for them to take power seriously, they either end up aligning with the rightists or they figure out they've got to reject liberalism and embrace a dialectical liberation as laid out by communists.