[jjk166]

Carbs and protein are water soluble while fats are lipid soluble, and your cells preferentially burn water soluble molecules before lipid soluble. There can be tons of energy in the form of fat available, but to access it you will first burn up your available protein. This protein is necessary for things like cellular repair, muscle growth, etc. You need to sacrifice this to access your energy stored in fat - an acceptable tradeoff when you're freezing in the middle of winter, but certainly suboptimal. Carbs on the other hand will burn preferentially before proteins, so you can have your cake and eat it too. Carbs can't be stored for very long, but they can be readily converted to fats for storage.

In the past, it wasn't so much that carbs were rare as calories in general were rare, and carbs were merely the most desirable. If you're an athletic hunter gatherer, you want as many carbs as possible for fuel so you don't have to switch over to your small reserve of fat and give up your proteins along the way. On the other hand in the modern day it's easy to get more carbs than we can burn in a short period of time so we have a lot of excess calories that get added to our emergency supply. Since we actually have to go through a good bit of effort to starve in the modern world, we never switch over to our emergency supply and thus it never depletes (ie we get and stay fat).

Of course you crave carbs after eating mostly proteins and fat - as far as your caveman brain is concerned, you are starving and need real food. It's just an unfortunate reality of our modern civilization that most of us don't have the metabolism to support a caveman's diet.