I considered the explanation was simply that diagrams created in UML are unreadable to lay persons.
Excessive use of visual jargon, and changing standards combine to make everybody a lay person, even those versed in a particular version of UML when confronted with a different version.
Not just forgettable, but vastly different meanings. And these meanings have changed between versions. You really have to get close to the spec to grok what is actually being conveyed.
The devil is in the details - if you have to understand the details like the right arrows for relationship types and visibility modifiers, then it gets much less trivial.
And if you're going to omit those details, then do you really need a standard for your adhoc boxes and arrows diagram?
Excessive use of visual jargon, and changing standards combine to make everybody a lay person, even those versed in a particular version of UML when confronted with a different version.