One of the reasons is that UML attempts to become a programming language.
By aiming to be able to generate full code actually UML tries to be a programming language. In my mind there is a big problem with a general purpose graphical programming language. In human history the written form of all languages evolved from graphical to textual (see also Visual Designs Don’t Scale). Alphabets proved to be more versatile and more expressive than pictures in capturing ideas. Try to describe any simple process in images. The funny thing is you still have to annotate the images with words. And the full text version with no pictures still gives you more details. Pictures prove to be good at sharing ideas and allowing people to visualize concepts. But in the end words are better at describing the fine details.