Good reasons for pugging have been posted. You cannot force people to group up.
And if the queues are not seperated you will likely lose a lot pugs anyway.
Ah, but no one is forcing a PuG/Solo-centric player to group. You still don't seem to understand... the current system allows Solo or Grouping up to 4 players. Essentially, Any one can play.
The same doesn't hold true for winning. If you see some one doing something that makes them win, why wouldn't you do the same? Is it truely unreasonable to expect players to try and get better?
Some of the greatest tricks and strategies I use have been learned from losing to them. We're not born with the knowledge that CC break should be saved if just mezzed by a stealther. It took me one time of being mezz capped from an early CC break to learn that. Like wise, it took me once of being pulled/mezzed on a firepit to learn I should do that on my Sin. Instead of crying how cheap it is to use firepits, I learned from smarter opponents and got better.
So why shouldn't the same hold true for grouping? I -can- break CC whenever I want, but I know I should wait. I -can- ignore the terrain and not use firepits, but I should use them. I -can- decide not to group up, but I should do so to increase my chances of winning. Simple right?
PS. "I don't wanna be forced to group" is still not a good answer for "Why is it so hard to find some friends?" but thank you for atleast trying to answer that one.