Why do academics frequently write very short email replies? Why do many academics write short replies to emails?

For example, I write Professor MK some polite email with salutation, etc., just polite and decent email.  MK then responds like this:


  OK, MK


Is MK so busy that they have no time to write, for example, this:


  Hello,
  
  It is fine. This is the correct approach.
  
  Best regards,MK


I do not understand this.
Beyond a certain point in their careers, academics (in particular tenured professors) are essentially a subspecies of managers.

And managers literally receive hundreds of emails per day. Some of these indeed require a more formal response, or a lot of work before you can even compose an answer. If you get 20 emails that each require half an hour of work, 50 more that only require a yes/no answer, and another 100 that are just FYI, you don't spend a lot of time on crafting replies to the yes/no mails... because the first category alone already is more than you can handle in a normal day, given all your other responsibilities.