UML, specifically statecharts capture a lot of behaviour unambiguously, though it is hard to draw by hand (due to its hierarchical nesting possibility, so some kind collapse/expand system is required to draw them in a sane manner).
UML, as others have said, is both too precise about things that are still vague at sketch time and took vague about things I want to be precise about to be useful. And once I have a real system, the real complexity in the system always makes for either an insanely huge diagram with lines everywhere, or an imprecise one full of lies and omission.
Trying to program with UML is basically like trying to write a system of 10k lines by first writing them all down, and only then beginning to try to compile it. It's not going to produce good results, not just because of the endless little errors, but especially the big ones that your could have discovered much sooner if you were using something more real than UML. UML is superficially easier to work with than direct code, but in depth, I think inferior to it.
Some of the notation is sometimes useful, but to use it as intended is crazy.
It definitely is, but are UML statecharts the right solution? Why not go the full nine yards and use a full-blown formal modelling environment, like TLA+? After all, using UML in a formal fashion would take more or less the same amount of effort as learning TLA+, which moreover:
* Is textual (making it easier to version-control and collaborate upon)
* Can be verified formally
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.
Because UML is usually used when you don't yet know or decided many details.
If you put them on diagram they will often turn out to be wrong, or even worse - artificially restrict the implementator's freedom for no reason.
"Why do we require 10 clicks to upgrade the subscription plan?"
"Because that's how it was specified"
This is not to say that blueprint-style UML has no value, it's just comparatively rarer, since it requires much more time investment. However, the two do not address the same needs.