why doesn't my body dump all the fat, since I'm eating a surplus of calories anyway?
That's a great research question. Normally, the body tries to maintain a happy "set point" of fat. However, different genetic populations (who may have historically gone through more famine) are more inclined to save fat than dump it relative to others.
Researchers suspect that there is some mechanism that may be going awry due to our modern diet/lifestyle. Fat makes a protein called leptin; the more leptin in our body, the fatter we must be. So why is our body not properly measuring how much leptin is in our bloodstream and thus thinking it is skinnier than it really is?
Answer that and you will both win a Nobel Prize and become a very rich person.
Presumably because carrying surplus fat has minimal cost relative to the evolutionary likelihood of famine. This is more obvious than the muscle scenario. For muscle, prior to the famine you get to do cool things like punch your neighbor and do pull ups. During a famine, you could consume your muscles as calories. Where is the cost?
During a famine your fat neighbor will survive longer. Storing fat is more efficient than building muscle. Maintaining fat is more efficient than maintaining muscle. And metabolizing fat is more efficient than metabolizing muscle. So while it's cool that you can do pull-ups, your fat neighbor will easily outlast you in a famine unless you're willing to eat him.
