This is a user to user forum, so we’re just users like you so we have no idea why Apple does what Apple does.
As others have suggested a person could use the present US holidays calendar as a guide, then make their own holiday appointments that repeat each year. They will just need to research some of them each year, such as Easter, the date of which is based upon the date of the equinox full moon.
There are others such as MLK day, Labor Day, Memorial day which are the ____ day of the month....such as third monday on January or whatever it is. Tax day can move from year to year based upon the holiday that’s observed in Washington, D.C. where the city and federal offices are closed in observance of Emancipation day. As well as when Veteran’s day is observed vs the day itself (same with July 4th and the Christmas holidays which may be observed on a weekday when the real day falls on a weekend.)
It’d just take some online research to find out exactly how each holiday’s date is determined.
Or as has been suggested there are plenty of calendar apps out there that may better suit an individual’s need.