From the airline’s point of view, having the next person right there ready to get in their seat is preferable to having everyone come up one at a time. This is why they have boarding groups. You usually see between 3 and 5 boarding groups because it’s a reasonable number between 1 and N (N being the total number of passengers). I’m curious how ~10 boarding groups would fare, but presumably there’s a reason airlines don’t do that.