Because there is a class of people in the US that does actually receive an unfair advantage over the rest of us, so many people deem that ideal totally impossible since they aren't having it handed to them, like some people in the US actually do.

Personally, I think the whole "standalone house, picket fence, one car per adult" stuff is dumb not because it is unattainable, but because 1) that's not really sustainable for several economic and environmental reasons but mainly 2) I grew up with that and it sucked. I couldn't go anywhere on my own since literally nothing was within walking distance, and driving was an absolute nightmare because of all the traffic. I want to have a nice apartment where I can go downstairs to a convenience store and a couple restaurants/coffee shops and bike to most of the places I'll need to go.

One legitimate criticism not of the "American Dream^TM" itself but of the way people treat it is that some, usually older, people will act as though it's a one size fits all solution to happiness, that getting married, having your statistical 2-3 kids, dog and cat in the back yard blah blah blah will make you happy as attaining that is "success," you've won, that's the ultimate. But many people don't want that. They don't want to settle down and they shouldn't need to. Mowing a lawn, driving your kids everywhere, buying 1 or 2 weeks' worth of groceries at a time and a long commute to work is not fun for many people. And the advantages of that lifestyle simply aren't worth it. The true american dream should be to help others and gain the life that you want for yourself, but too many people treat it as some tangible thing, like a diploma to say you've graduated young adulthood and are now ready to buy a motorcycle for your midlife crisis before calming down.