The issue is that the endowment is usually restricted. Donors can place restrictions on how their donations are used, such as specific departments or specific scholarships. I’m not sure about the specifics of Harvard’s endowment, but many endowments have withdrawal restrictions (only X% can be used per year) and/or restrictions on whether the principal can be withdrawn. If the principal can’t be withdrawn, then they’re limited to the income generated by investing the donations
The public universities are probably going to roll over for the admin’s demands, especially when many of them face funding cuts from their state legislature as well. Harvard is taking a stand (thus getting much more focus from the admin) because they may actually be able to survive this battle using their endowment. Other universities can’t say the same