Because the earth’s rotational period is not exactly .250000 “off each year.
That .242 bit requires we “subtract” (well, not add a Leap Day) every 1000 years.
No, the .242198 means we don’t add a 29th of Feb on the century mark, except for each fourth one. That’s why we got a regular leap year in 2000, and most people alive at that time never saw the skip of one in 1900. Most people alive in 2000 won’t see the skip of one in 2100.