I suspect these are really only happening for the little kids whose parents might not otherwise have a party for friends cause they are 2 and 3 years old (we only did family parties at that age).
That wasn't the case back in the Dark Ages when my kids were young. It was also in part because often the kids your child saw and played with during the week had parents who weren't friends with you. So this gave the child a chance to celebrate with those friends. They were sometimes useful if a 3 year old started talking about
"my birthday party" because uninvited kids just thought the child was talking about the birthday party they did attend. Very useful for avoiding hurt feelings!