Because you don't want to deal with the idiots that respond to CL postings. I had four people flake out on me in a 24 hour span. These are people who I spent quite a bit of time going back and forth with negotiating price and agreeing on when and where to meet. Then I would text them before heading out to meet them to confirm everything just to have them change their mind or just fall off the face of the earth. All for $50 more than what Gazelle was offering for my old 4. Not worth the time and aggravation in my opinion.
Can't say I have ever had this kind of experience. I guess it depends on how you negotiate and the type of people that contact you.
With the exception of those first 2 scammers who contact you and ask for shipping and drop the whole dramatic story, selling on CL has been smooth for me.