All you need to know to understand why the calendar in current use is screwed up, is that it originated in
1582, introduced by
Pope Gregory XIII as a reform of the
Julian calendar to better align with the solar year and fix the timing of Easter. It has not been updated since.
If that is not enough information, about 50 years after the introduction of the Gregorian Calendar by Pope Gregory XIII, the astronomer
Galileo Galilei was placed under house arrest for declaring the Earth revolved around the Sun (
heliocentrism) after his trial by the
Roman Inquisition in
1633, following his publication of
Dialogue Concerning the Two Chief World Systems, and remained confined until his death in 1642. He was found "vehemently suspect of heresy," forced to recant his views, and sentenced to live in his villa near Florence for the rest of his life.
The calendar in general use by the public today originated while the Earth was believed to be the center of the solar system with the sun, planets, moons, and asteroids rotating around it.
Of course, to put a more accurate and useful calendar into public use now would require a dozen world conferences, thousands of committee meetings, and it would still be like trying to get the public in certain countries to use the metric system. People are always opposed to what they don't understand and reluctant to accept any changes from the way it was always done, even when that is all wrong, and the new system would be far more accurate and less complicated for everyone.