I have attached a screen shot from google image search on modern headshots - as an example...
Which images draw you in to look at the person , or even to linger on a persons image among a sea of other peoples images? We are inundated daily with tons of social media selfies/avatars and headshots as we surf the web.
The individuals who really created the idea that a fabulous headshot is a necessity, were actors, and entertainers.
People who needed to get noticed ... people who know that the casting directors might look through 3000 images in a day ,
looking for just the "right" look for a role. They all know that they NEED to do something to stand out in a crowd.
So, recently photographers have started to follow their trends in the headshot business...
In the sampling below, we see how a lot of headshots are being done by the most elite of photographers nowadays ...
and the reason for cropping in so tightly , isn't because we want to zoom, in or be artistic.