Unfortunately, that's failing to look at the big picture.
When every guide is "kill scrakes solo!" "how to kite (alone)" "how to solo hans!" and everyone specifically practices only how to play solo and not learn how to play in a group, what they're really learning is that they don't need their teammates to stay alive, and in fact those teammates are just complicating the cut-and-dry soloing plan they've memorized and getting in their way. (and stealing THEIR dosh and xp, and stealing armor and ammo pickups that THEY need to beat the wave alone, etc.)
So people run off from the group, they let their teammates die, they do selfish things like grab armor when they've got 95% and there's someone else running towards it with 0%, etc. Far more acts of multiplayer stupidity can be attributed to the selfish mindset promoted by the community's obsession with soloing strategies than it can to mere newbie ignorance (especially when the incompetent players in question have perk levels over 10)
The goal of most of these lone wolf players isn't for the team to survive the wave, it's for them personally to outsurvive the entire team. Once they're the Last Man Standing, they are rewarded with ALL of the xp, ALL of the dosh, ALL of the glory, and the enemy AI becomes more predictable because it isn't reacting to anything but them.
I'd say most of this could be summed up by "pugs will be pugs".
However I'd also add that it will always be a problem when the game allows (or actually even encourages now) kiting to an extent. If the zerk as a whole and medics speed/armor bonus didn't exist, people would be a LOT more likely to stick together.