why do tsumegos have exactly one solution?

[Cassandra]

在中国古代死活题中，强调的是题中最精彩的变化或容易忽略的陷阱，而不是唯一的解答。
In ancient Chinese life-and-death problems, the emphasis is on the most brilliant variations or the easily overlooked traps within the problem, rather than on a single solution. (by DeepL)

This is also true for Inoue Dôsetsu Inseki's Igo Hatsuyôron 120. There are many solutions that do not follow the line of Hideyuki Fujisawa's "classical" solution.
井上道雪·因石的《围棋初论120》也是如此。其中有许多解法并不遵循藤泽秀行“经典”解法的思路。(by DeepL)