They cannot write because they are no longer expected to write much of anything in schools. Also, they do not read much written in standard, grammatical English so they have little sense of what that is.
I have seen BS degreed people struggle for hours to write a simple paragraph describing a lab procedure. This kind of writing need not be lyrical, but it must be absolutely clear and unambiguous. Most younguns now believe that technical writing means using a lot of multisyllable words and as much verbiage as possible.
I believe that if you cannot write about something, then you do not understand it, either. There are a frightful number of degreed people who know a good deal less than they think.
Just for giggles, ask a kid to write instructions on how to make a cup of tea for someone who has no idea how to do it—an alien, for example. Very few Americans can competently write exposition, but it is critical in many settings.
Just for more giggles, ask a kid which is smallest
a. .19
b. .090
c. .009
d. .0091
For some strange reason about 20% or more will get it wrong and tell you, “Everything works backwards to the right of the decimal.” Whatever that means.