With support being my main role let me give you a little bit of clarification on how we support (and ADC's mains) see the situation:
Our roles are (apparently) the least played, and thus there is almost always no wait time for us to get into a queue. As soon as we hit the ready button, we find a group, if we leave said group, we instantaneously find ANOTHER group waiting for us, wanting us, needing us.
This means a few things:
A) We are not all that likely to wait around in a near empty group, when all it takes is a few clicks and a few seconds for us to find a already full group ready to go. You may end up waiting longer, how ever we do not.
B) Since we do have the option of so many groups wanting us it allows us to cherry pick our teams. Say I'm queuing up as a Soraka support: I am not tanky, and thus I am going to want at least a tank in the jungle or in the top lane in order to balance out our team. If the group I get put into is a Zed mid, Yasuo top, and Kha jungle, I am not going to stay because they all are squishy, and they are all AD. I WILL find a new group and it WILL only take a few seconds.
I hope this helps clear up some of the confusion you felt as to why us supports and ADC mains don't stick around in every single queue we get put into,