A family of Microsoft word processing software products for creating web, email, and print documents.
All I can tell you is that it is a design decision for which the UI provides no control. My guess is that it is designed this way in order to provide an accurate indication of what the comment pertains to.
I'm not sure what it is that you might prefer but don't hesitate to submit your suggestion/feature request to the development team by using Help> Feedback in Word's main menu.
FWIW you can select as little as a single character [even a single space or period] in order to have the highlight applied only to it.
As for the location, in my experience the initials always are placed at where the insertion point (cursor) is at the time the Comment is created or immediately following the selected passage..