What I was trying to say earlier is mostly just that it's more fun and interesting when there are many girls than when there is just a soloist. With groups, they can do better and more unified dances (vs. soloist + back dancers), we can complain about line distribution and get excited when our favorite underdog gets a solo line and it sounds awesome, we can discuss how each girl looks in the outfits for the new single/album/etc, we get concerts and performances with shuffles, etc.
I like idols because it's fun to see their friendship and teamwork, so overall I enjoy groups more. (I know there are friendships between different H!P acts, but they work with their group members the most.) People say things like how one-member generations in MM seem lonely, and soloists seem even lonelier.