To add to what others have noted about it not being a diagnosed condition and more a symptom of an undsrlying cause, I also want to add it can be used in a situation with MULTIPLE underlying causes where "Exhaustion" is just easier.

Say you have someone who has the following issues: Disturbed or broken sleep cycle due to undiagnosed sleep apnea, malnourished becuase their diet isnt complete/varied enough, high stress fatigue due to constant financial concerns, hypertension, blood sugar approaching diabetic levels.

You know, a fairly common American adult trying to work a 9 to 5 thats more a 5 to 9 while being worried about everything under the sun.

One of the effects of all this is the person is regularly going to be bone tired due to the lack rest, high stress, and poor diet.

"Exhaustion" covers the resulting condition of all of those underlying factors because its not one diagnosis, its a lot.