Because the more mathematically rigorous and pure a description is, the more likely that your programmers won't carefully read the spec, or won't fully understand the exactly meaning of the UML arrows.
It doesn't help that every UML diagram I've ever seen is ugly. There are always way too many jagged lines, and seemingly random attachment points. (Example: https://tallyfy.com/wp-content/uploads/2018/02/Class-Diagram...). In the little triangle, one line goes straight down, and one goes to the side. Is going to the side special? Or just a random artifact of the diagram? Without being an expert, it's hard to know.
With written-language descriptions, it's easier to be rigorous enough for all programmers.
It doesn't help that every UML diagram I've ever seen is ugly. There are always way too many jagged lines, and seemingly random attachment points. (Example: https://tallyfy.com/wp-content/uploads/2018/02/Class-Diagram...). In the little triangle, one line goes straight down, and one goes to the side. Is going to the side special? Or just a random artifact of the diagram? Without being an expert, it's hard to know.
With written-language descriptions, it's easier to be rigorous enough for all programmers.