Why is the solar term in the lunar calendar a fixed month and the solar calendar a fixed date?

As a traditional solar term in China, Tomb-Sweeping Day is calculated according to the annual vernal equinox, which has a solar term every15th day, so Qingming is mostly on the 5th day between April 4th and 6th in the solar calendar. It is not fixed on April 5 of the solar calendar, nor is it determined by the lunar calendar. The vernal equinox changes with the rotation of the earth. The earth's daily rotation is not 24 hours, but 23 hours and 56 minutes, which will be reduced by one day every four years, thus affecting the time of solar terms. According to this rule, April 4th happened in Tomb-Sweeping Day in 2008 and 2009. On April 4th, Tomb-Sweeping Day appeared in 2004 and 2000, and on April 6th, Tomb-Sweeping Day appeared in earlier years such as 1939 and 1935.