I have a few really old unfinished projects, but I generally finsih everything. I often pause to make a quick quilt for charity when I am in the middle of something ticklish. Last year I had a physical problem which made quilting my tops hard for me. I got 18 to that point, but I have those all finished and delivered now. Generally I have around 20 projects in various stages of completion.
Some I am in no hurry to complete. I started a rather tricky one for a niece who is currently being a pain. I'll finish it, just not right now. I have about half the hand appliqued blocks finishe for my DGDs wedding quilt. As she is 10, I don't think she'll be needing it soon.
Another reason to have various projects is that they require different types of work. Right now I have one embroidery project and one binding project, as well as a couple of active tops and one quilting progject.
Many people I know complete each project before they begin the next. It works for them. It would not for me.