A promise is a comfort to a fool

Why do people say they will do something, realize they cannot, but find it hard to update you?
I try my best not a make a promise I cannot keep.  If I cannot keep my promise, I think it is the respectful thing to do and let the other person know as soon as possible.  Sure there may be disappointment, but it can be dealt with. No one is perfect and things always come up.  But, with a little courtesy, many issues and disagreements can be avoided.