Because in addition to your own carry on and settling, you have someone else's to attend to. A parent with a 3 year old has to get the kid's bag into the bin along with their own, help the kid buckle up, soothe any worries, find the juice box/stuffed animal/coloring book to keep the kid happy... and it's all easier if you don't have people breathing down your neck or shoving past you because they're impatient.
I absolutely agree on no extended family and the rest of your stuff, but there are good reasons why boarding early is attractive to parents of small kids, regardless of whether it's fair or not.
I paid for EBCI. I am also traveling with a 5 year old. I definitely think the people who pay for EBCI should board before family boarding. While I appreciate the courtesy of family boarding, the part of me that shelled out $100 would be really irritated if I was traveling without a little kid and got bumped by someone with a first grader. 6 is awfully high for a neurotypical, able-bodied kid to qualify you for family boarding.