Even taking a away the race issue the western left does have a problem with thinking the only social mobility that matters is people going from the poorest life with no family or community sort at all right to the top. It's far, far more common for social mobility to take a few generations where poor parents decided to focus on their kid's eduction and attitude, the next generation is almost middle class and can actually invest money in pushing their kids on and grooming them to fit into a different class and then the next generation has the skills and funds to support their kids learning whatever they want and running one loss making start up after another in their garage for years.

The trouble is in an individualist society if a person dies without getting their dreams that's seen as an out and out failure no matter what good they do for those they leave behind and that last stage is 100% seen as well off parents excluding poor kids, and giving their undeserving brats a leg up rather than a family which has worked hard and smart for generations to finally give their kids a chance. Indeed most people who do rise up from the top to the bottom in one life time are often quite unhappy because they feel they have to put on a persona to fit in with their new class and can't be themselves because social skills and networking is a important as brains when it comes to success and is so much easier if you've been brought up to do that.