Because freedom has failed them. Or, at least, "freedom" as defined implicitly by the author to mean "only a specifically enumerated list of negative freedoms that primarily benefit the wealthy" and "the ability to cast a vote which fundamentally does not matter" -- remember the adage "if voting changed anything, they'd make it illegal".
Millenials don't feel particularly free to speak when they live in permanent debt, begging the wealthy to gift them with an unpaid internship and praying they don't get sick. When they do speak, their voices are drowned out by billions of dollars in propaganda campaigns. Yet they are told "this is freedom" and you're surprised they value it less?
If you asked them whether they want "freedom" of course they would say yes. But it's ridiculous to say a homeless person is "free" because the cops don't arrest and harass him for speaking his mind. It's ridiculous to say a millenial is "free" when those of them lucky enough to get jobs have to toe the line and obey every command from their employer, watching what they say on social media, accepting the most egregious reaches of corporate power into their personal lives so that they have a prayer at one day paying back their insurmountable debts.
Millenials, unlike the generation before them, recognize that material conditions are of fundamental importance and that any notion of "freedom" that ignores them is not worth much. The "freedom" to buy a news organization or bribe a politician with infinite dark money is just enshrining corporate power. The "freedom" to buy healthcare is meaningless without the means to also afford it. Millenials see through the very shallow veneer of whines about "freedom" that do nothing but obscure blatant attempts to preserve current power structures.
Consider tech contracts: companies write contracts so that they own what you think in your off time. They write contracts so that if you decide to quit, you cannot legally work for years. That is not freedom, it is the opposite. Yet the libertarian screed is "well, in the abstract, you have the freedom to make that contract or not as you see fit"; what consolation to those who need a job to pay rent or buy food.
This attitude shift is mostly definitional semantics and is just a natural result of decades of libertarian/conservative weasel-wording around "freedom" so that it is a meaningless platitude that expresses nothing more than "a principle which benefits those with means".