Consider how back-to-front boarding works in real life. When everyone from the back enters first, they all stop to stash bags while people in front rows are still getting settled, blocking the aisle. That creates a bottleneck that actually slows the whole process down. Group numbers aim to break up the crowd so not everyone is fighting for space at the same time. It's messy, but removing groups might make the shuffle worse because everyone crams in at once. I guess airlines keep it because some order, even if annoying, beats total chaos lol.