Why do academics frequently write very short email replies?

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.

Is hello acceptable, or should it be more formal (with or without academic titles)? This is a matter of (sub-)culture. Once you know what to expect, you can effectively communicate. This style is minimalist, but it tells you everything you need to know.

If this bothers you, think of it this way: MK is not only saving their time, but they are also saving your time. By using this style it minimizes the time you have to spent reading their message.

My experience as a professor is exactly the opposite. I write full emails, and most often students' replies have no heading, nor greeting, nor signature.