Other non-profit entities (such as private foundations) are required to spend a certain fraction of their money each year or pay taxes. Universities are exempted from that requirement.

from [0]: "Harvard targets an annual endowment payout rate of 5.0 to 5.5 percent of market value. The University's actual payout rate has fluctuated over the past 10 years, from a low of 4.2 percent in fiscal year 2006 to a high of 6.1 percent in fiscal year 2010."

[0] https://www.harvard.edu/about-harvard/harvard-glance/endowme...

[1] https://www.irs.gov/charities-non-profits/private-foundation...