It might seems strange at first that freelancers are refusing well paid work ahead of time. But when you are billing every minutes of your work, it becomes clear that saying 'yes' can cost you more than saying 'no'.
With every 'yes' comes a commitment, and you might be tempted to think that you are doing a two way commitment. Actually, most of the time the commitment is one way, that is : if your customer does not need you after all, you will not work. That is the definition of freelancer : with time comes uncertainty.
The problem is that since you made a commitment you closed many doors that opened along the way. And if you are not aware, it might cost you a lot. (and not always in terms of money)
Commitment is not something to take lightly, and, as a freelancer you have to know that this is often a one way commitment. So protect your time it is your most precious asset.