It is very important to emphasise that liberalism sees failure and inequality of outcomes as inevitable and important. Inequality encourages people to work hard. When more competitive firms buy less competitive ones using their profits it ensures that resources go to where they are the most productive. Innovation and progress come from the drive to ‘get ahead’. Equality of opportunity is critical in this paradigm for ensuring competition. But close equality of outcomes is counterproductive. This is the most fundamental point of divergence between liberalism and progressivism.
To be fair, you will likely never hear anyone, no matter how left-wing, actually claim to be in favor of equality of outcome. Their argument will instead be that there isn't any true equality of opportunity because of...anyone?...anyone?...privilege. As Thomas Sowell put it in his excellent A Conflict of Visions:
So long as the process itself treats everyone the same — judges them by the same criteria, whether in employment or in a courtroom — then there is equality of opportunity or equality before the law, as far as the constrained vision is concerned. But to those with the unconstrained vision, to apply the same criteria to those with radically different wealth, education, or past opportunities and cultural orientations is to negate the meaning of equality — as they conceive it. To them, equality of opportunity means equalized probabilities of achieving given results, whether in education, employment or the courtroom.
This formulation, I think, gets more to the heart of the matter. What makes the progressive vision so incoherent is this idea that there could ever be a world without "unfair" advantage, whether naturally or by means of the most precise social engineering. We can all feel sympathetic to complaints about a world in which some people just can't seem to catch a break, while others have access to every resource they could ask for. For some people, though, that understandable sense of sympathy stops them from taking a clearer look at the logical fallacies that follow. Advantages can accrue to individuals through something as simple as growing up in a stable, loving family who instill a love of reading books. It's simply impossible to quantify, let alone fairly allocate, all the countless variables that go into making one individual more talented or successful than another. Anyone who had the time and ability to set about eliminating all of life's inequalities and contingencies would find themselves needing to play God and redesign the world from scratch. Lacking that option, the next best thing would be to monitor and control this one like Big Brother. (Lacking that option, they're forced to settle for incessantly complaining and crusading.)
Of course, it's highly doubtful that any progressives actually take the logic behind the rhetoric that seriously in the first place. They'll happily accept a permanently unfair world as long as they can hold political power in it. Perhaps the rest of us could merely stop granting them the assumption of their moral respectability.