Why don't languages or IDEs support attaching descriptive metadata to variables?

You can put arbitrary metadata on functions, classes and other things that are basically objects in in Python. If you write

   def somefunction(x):
      ... body ...

you can later write

   somefunction.specialmetadata = "a value"

so what you can do is fairly unlimited. You can't write, however

   x = 4
   x.metavalue = 3

as the int isn't a general object. Even if you did the metadata would apply to the value (the '4') and not the variable (the 'x')

Clojure supports metadata:

https://clojure.org/reference/metadata