Something to consider is that your body relies on blood glucose as its primary energy source. During starvation, glucose levels are severely depleted. This triggers your body to start using stored fatty acids. All remaining glucose is reserved for the brain to use.
By removing blood from your body and moving it to your stomach, you’re essentially moving that precious energy to an organ that can’t as readily make it available to the tissues that need it.
Thanks to the thermic effect, it also takes energy to digest and metabolize food. You’d be expending extra energy to digest the blood that was already in your body, where it was perfectly content carrying usable energy where it was needed.