I would argue that opportunity is the biggest factor, and most places in the world don't give you a lot of opportunities.

There are a ton of situations that you can have, just in the US, that can stop your social mobility and/or keep you in the poverty trap.

Not being able to afford school; not being able to afford to survive while you go to school; not being able to access the health care you need to maintain enough functioning to get into a higher paying field, learn a trade, or go to college; being in a situation where you feel that you need to take care of a sick relative or younger siblings because you've been raised to put yourself second for the needs/convenience of others; becoming a single parent too "early" for any possible reason...

Someone I know has their extended family paying $50k out of the $80k they need to go to their university every year, and the rest is covered by scholarships. They have OCD and generalized anxiety disorder, but they get the health care that they need to manage that. You may see how this person might have a lot more opportunities than most people to "get ahead", so to speak; and I think that a lot of people would be as or more successful than they are in the same position. They just don't get the same kind of support. Hell, even this person's sibling doesn't get remotely the same kind of support.

IMO this is where the government should come in, but obviously (at least, again, in the US) it doesn't. I know a whole bunch of motivated people who can't get ahead, and the reasons are almost always systemic. I also know some people who don't want to get ahead, so to speak, but they are few and far between and they're also my immediate family which is made up mostly of grifters.