...they just can't always find an agent.
One of the things that held me back from attending critique groups in the beginning of my writing was the idea that it made no sense for me to "learn" from other people who have not had any success getting their own work published.
Yes, I know, there are published authors at critique groups, probably quite a few. But let's face it, the majority of participants are not. At least, that is the case around me. And none of them are JK Rowling :).
I've made it my policy for a long time to never take advice from someone who does not have more experience/success than I do with something. I wouldn't take financial planning advice from someone who lives off credit cards, or parenting advice from someone with no kids. I take nutrition and exercise advice only from people who are more fit than I am. You get the idea.
But writing is different. Even someone who writes atrociously may spot a typo you've missed, or notice that you over-use the word "has." Or, maybe they write just as well as you do, but are in the same boat and despite obvious talent cannot find an agent or publisher.
I was lucky enough to attend a couple of writers conferences and learn from some real pros, like Bryan Davis. But I've learned just as much from my critique group that consists of not one single traditionally published author. Their insight has transformed much of my writing and helped me hone pieces that were ultimately bought by periodicals. They have offered tremendous advice on my novel-writing as well.
And I don't let my rejection letters stop me from helping my fellow writers. I was a teacher before I had kids and now I homeschool--so I love that I can use my teaching skills during this process. Writing has introduced me to a whole world of teaching and learning that I could never give up.