I can think of several reasons why this would happen.
- The divers are so unskilled that they're struggling just to maintain their position in the water. They're using up all their mental bandwidth just trying to keep from rising or sinking, and don't have anything left for anybody else. It's an "every man for himself" situation.
- They were never taught how to be a dive buddy. Sure, they may have gotten some platitudes about how this is what you're supposed to do ... but nobody ever bothered saying "here's how you do it". This is an "I don't know how" situation.
- The instructor doesn't take buddy diving seriously, so the students see this and figure they don't have to either. This is a "why bother" situation.
- They figure the rules don't apply to them. This is the "who, me?" situation.
Good call ... the rule for kids is that they are only allowed to buddy up with a parent or someone the parent specifically designates.
It sounds as though your instructor doesn't take the buddy system very seriously ... therefore his students won't either. This was my impression reading your initial post, when you were discussing the class diving as a group. One doesn't learn buddy diving that way ... one learns herd diving. Herd diving is where everyone dives as an individual and focuses on the herd leader (in this case, the instructor).
The proper way to teach buddy skills is to take only two divers at a time on the dive, and make sure they know that they are the team ... the instructor or DM is only along to observe, and to provide feedback afterward. Prior to that dive, the skills of cohesion and communication need to be discussed. Cohesion means "here's how you descend together, here's how you dive together, and here's how you ascend together". Communication means "here's how you stay in contact with each other during the dive". This includes hand signals, the use of lights (when appropriate), positioning, and body language. It also involves some behavioral changes ... like learning to turn your head rather than relying on peripheral vision (which you don't have with a dive mask on) or assumptions (that your buddy's going to do what or be where you expect them to). If these things aren't taught, then you don't have a buddy at all ... you have a liability in the water who's likely to do unpredictable things and keep you too stressed out to enjoy the dive.
The skill that most makes you a good buddy is also a skill that is most needed for solo diving ... that would be good awareness of what's going on around you. There are many excellent reasons to solo dive ... lacking buddy skills isn't among them. At issue is that if someone's too lazy to work on being a good dive buddy, they're unlikely to have the self-discipline needed to be a good solo diver.
The problem with this approach is that it tends to create a dependent diver ... someone who hires someone else to "take care of me". That may work out OK for easy tropical dives, but for more challenging conditions ... even at your local quarry ... it's not a good idea.
It's far better to limit the scope of your dives for a while and work on developing the skills needed to dive as a buddy team. Being a dive buddy is a commitment to a certain amount of effort and self-discipline. When you're diving with a loved one, the motivation to make that commitment is very strong. That can be a positive thing ... as long as you remain within the limits of your training and comfort zone.
Some time back I wrote an article intended for those who wanted some guidance in
how to be a dive buddy ... some of the newer divers out there may find it useful ...
... Bob (Grateful Diver)