I have, on occassion, worked on more than one project at a time. However, I find it distracting. For me, it is better to focus my thoughts on one project at a time. I mean, do you have to believe in the story or do you just rewrite it because you are being paid. If not, you won't have the passion to do your best work.
Money is a part of it.
About 5 years ago a studio wanted me to rewrite a script. I read it and said no. They then doubled the offer. Again I said no. I said yes.