I feel like all the points you raise could be replied by : if you do not like it, no one is forcing you into doing it.

It is my understanding that people do this for fun - to take the occasion to get into a new language and/or exercise their problem resolution skills.

Personally, although I love coding (it is a passion), after a whole day of coding I do not feel the energy to partake in a coding event. And during holidays I am busy doing other stuff. So I do not participate in the Advent of Code. But I am still glad that the event exists for people who enjoy it