A few reasons.
Part of it is that wealth never really got passed down to us. Boomers got money and houses from their parents. Some GenX got money and houses from theirs. Millennials increasingly have family that live longer because of modern medicine (depending on your family, good or bad) and increasingly have family that burn through all their money because they want to buy mypillows and trump watches. Not to mention how many of us went zero contact because of the aforementioned trumpism.
Which, combined with multiple economic crises (how many of us saw the job market and said "I'mma gonna get a PhD and deal with that later"?) means that millennials have kinda been struggling to even get their lives "started" well into their late 30s. Hard to become a professional politician when you are always a few months away from poverty.
And the last part is... 9-11 kinda fucked us. Basically starting in the early 00s it became a mortal sin to criticize the military. Even today, we are allowed to criticize "the military industrial complex" but NOT the brave men and women who are on our streets oppressing people and helping ice round up brown people and protesters.
So by the time a good chunk of millennials reached the point where we are stable enough to fight and the world isn't going to villify us for doing so... we also have responsibilities. In your teens and twenties you can talk about burning it all down and quitting your job to live in a tent for 14 months while you smoke weed and protest and smoke weed. You might have kids that you need to give a chance at a better life. A partner with medical concerns. Or just a cat that would slit your throat in your sleep if you moved into a tent.
So all the pushes for progressive politics and the like get us labeled as "traitor neo libs' by the kids who want to live in tents and film tiktoks and protest and film tiktoks. And when you get told "okay boomer" when you are in your 30s... it is REAL fucking demoralizing (this is where some jackass replies "okay boomer").