Interesting questions/ riddle. I also wonder why?🤔

The short answer is: "Second Century BCE Roman political machinations."

Christopher, no that is not the reason.

The evolution of the modern western calendar ("Gregorian Calendar") is well described in these Wikipedia articles:

- https://en.wikipedia.org/wiki/Gregorian_calendar

- https://en.wikipedia.org/wiki/Roman_calendar

Calendar evolution seems to have been guided by a chaotic intersection of religious, social, economic, and political issues, with no one factor completely responsible.

. Additionally, from personal observation when discussing "significant calendar modifications" with family members, and from historical records of protests around such changes, we find that many average people have "Reified" abstract calendar concepts (those beyond the objectively observable components of Solar-Day, Lunar-Month, and Solar-Seasonal-Year), and get very confused and upset in response to such changes. (Eg: British Calendar Riots of 1752.)

. Also, many highly-religious people believe that their calendar was created by god, and consider any organized social attempt to "change the calendar" as insane blasphemy. (Eg: Biblical Genesis imputes that god created the 7-day work week.)

. When combined with the basic astronomical problem of reconciling a solar year with approximately 12.368 lunar "moonths" and 325.25 terrestrial solar days - everyday mathematical disharmonies that have perpetually irked civilized humans - it would seem to be a near miracle that the modern western calendar has been so relatively stable since 1582, and a sad necessity that the calendar year begins 10-11 days after the Winter Solstice.

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(Caveat: these are my considered opinions based in reliable and well-reasoned encyclopedic sources; I have no recognized academic expertise in this field.)