Inexperience does add to the risk of someone getting injured, but the real reason is that everybody has a different way of doing things. If a group of people ride together a lot, and know each other's habits, generally they're not the ones that have issues. Motor cops in one city don't have any issues when riding with each other. Throw in a different city though? Depending on what type of escort they're conducting, it can get hazardous. And, generally, they're not inexperienced or untrained.
I ride staggered when I'm with most of my friends. My kids and I will ride side by side. 1) We have Senas and can communicate what we're going to do and any potential problems we see ahead. 2) We've ridden together for years and know each other's reactions. 3) We don't stay side by side on twistie roads.