I nearly quit after so many of my brews were poor quality. Started to seem pointless to spend the time making it, get halfway through a keg, get tired of it being crappy, and then dumping it.
Brewing known good recipes (vs making them up on the go at the LHBS), and learning about closed transfers, brought me back around. Suddenly I had delicious beer I was excited to have a glass of.
I think I'm echoing the earlier ideas that some people don't end up good at homebrewing and so they drop out. I don't blame them. Some probably even tried a lot and didn't quit quickly.