The vast majority of the endowment isn't money (dollars in bank accounts). University endowments work like private equity funds, most of the funds will be invested in assets, most of which hardly liquid enough to reasonably convert them into cash on short notice. They could try to borrow money against the valuations of those assets, but it's not sane to take on debt in order to sustain a level of expenditures that was adjusted to a much higher level of income (true more generally). Especially when the alternative of temporarily scaling back expenses is relatively easy.

I am very skeptical that endowment funds are as illiquid as you claim. We're talking about amounts less than 10% of the total portfolio size annually.