If we are not fasting for the cleansing and healing and/or spiritual benefits and are primarily fasting for weight loss, i see your point that we might come to a place of "why bother" when the temporary portion of the losses very quickly return. People who love fasting generally love it for many reasons besides the speedy temporary weight loss it brings.
The actual weight loss that stays off, or at least can stay off (fat loss) is not nearly as impressive as the overall loss which includes all the temporary displacements which as you can testify will return to your body within 2 or 3 weeks.
Where actual enduring fat loss is concerned, similar results can be obtained through healthful nutrient dense diet and exercise... and for individuals not interested in the amazing benefits unique to fasting this would be another way they could go.
I made recent replies to a couple of similar concerns here: