I also have several going on at once. It provides variety.
if I only had one, I think I'd quit sewing because the minute I got bored with it, wanted a bit of variety, it would begin to be a chore. By having several going on at once, you can pick and choose. If choices are painful for someone, this is not the way to be, however.
I can understand those who only do one at a time, but I can't think like that very well.
I suppose everyone who can only do one at at time also finds it impossible to do long term projects like "block a month" and/or "the little houses" project, right?