Why don't some U.S. colleges celebrate American national holidays?
The observance of holidays in the United States is a complicated issue.

Holidays declared by the federal government are generally (but not universally) observed by state governments, which oversee public universities here. Private institutions, on the other hand, have no obligation to observe those holidays, and can choose to close or remain open according to their own decisions about what best serves their students’ needs. For example, my undergraduate institution did not take any days off from the first day of classes in the fall semester, but did provide a week’s break corresponding to Thanksgiving. It’s an unusual arrangement but perfectly legal.

It should also be mentioned that while universities have core “business hours” where faculty are expected to be present, in reality, faculty largely choose their own working hours.