Underneath, people are overwhelmingly just in it for themselves, and judge others by how closely they align with their personal set of "whats best for me" ideals.
Collectivism means the subordination of individual autonomy to the governance of the collective according to the needs of the collective. You’re a cog in a machine and your purpose is to serve the collective—starting with your family and radiating in rings out from there. I’m not sure Americans can even understand the collective mindset, much less practice it.
In my collectivist culture, the answer to those questions is "just deal with it." That's the bargain of a collectivist society. The collective will support you, but in return you owe the collective a complex web of social obligations from birth. I happen to think it makes sense,[1] but I'm not trying to persuade you to live in a collectivist society. I'm just explaining the concept.
[1] I'm married to someone from the polar opposite culture: an Anglo-Protestant from the west coast of the U.S. She once explained to our kids that they didn't have to give family members hugs if they didn't want to. She called it "bodily autonomy." I found this concept extremely bizarre.
I'm speaking about 20 years ago, when getting any kind of peer or social circle respect had the prerequisite of subscribing to socialist utopian ideals, and it wasn't something that was hard to foster in America's dead-end job work culture (which is where you work when you are young). This is urban/suburban America, where most people live.
From what I can tell this was the same with Boomers (they were the OG hippies afterall) and I see the same ideas in today's crop of young people.
The youth however hold little sway over the direction of the country, they're not actually that invested, so by the time they are having an impact, many have already received their first shots of the euphoric side of American capitalism, a career that gives them power and money (after years of wading through dead-end/entry level hell).
American millennials were hyper-individualistic and rejected socially prescribed roles even when they were young. What they wanted wasn't collectivism, it was a higher status within capitalism. Which is why, as you observed, the sentiment evaporated once they achieved that status. I'd make the same point about Gen Z. They want to think they're socialist and collectivist. But they all want to be online content creators and influencers--jobs that only exist in hyper-consumerist, capitalist societies!
This is not a criticism either of collectivism or millennials, by the way. I think Republicans screwed up the concepts during the Cold War era by successfully labeling Democrats as collectivist. What you have in the U.S. is more accurately described as two strains of libertarianism, one that emphasizes social liberty and the other that emphasizes economic liberty.
