It's not the hard to understand. The people "screaming" at each other in general are both DPS. They are looking for a group that already has a tank or healer. They don't want to be in a group with 3 DPS and no tank or healer. That's why people join and then leave, probably because they saw you only have DPS. That's the same as not having a group, you can fill DPS instantly whenever you need to. Usually your odds are better if you continue spamming to try to get into a group that already has a tank or healer.

They also don't want to lead the group, they want to join someone else's group so they don't have to spam any more because it's the leader's responsibility to fill out the group. That's why they don't take kindly to your suggestion that they invite that other DPS that is also looking for a group. It's not lack of thinking, it's just being lazy.