It doesn't burn only the carbs at first completely and THEN the fats and proteins, it blends in fat (and probably protein) burning but not enough to get into ketosis. I don't think it's an all-or-nothing thing, otherwise getting into ketosis wouldn't be gradual (it is gradual). Remember that the body craves carbs because of a few bodily vessels (such as the liver, notably) that cannot run off anything but glucose, so it will try to hold onto them... unless you force it off them. Also remember that most people go into a little keto overnight since you run out of glucose storage if you don't eat between 6pm and 9am or so (see, "break-fast"?)
I'm probably not explaining it 100%, just read The Case for Keto and it makes more sense
So I have never been much of a drinker, but was raised with tons of sugar and am a hard core sugar addict. I went on a low carb diet to try to combat migraines and limited myself to about 100 carb grams a day, not even keto level. Within a week or two, my cravings disappeared. POOF. Gone. If I cheated and had too many carbs, the cravings came back fast. I think it has to do with insulin controlling other hormones that trigger hunger and such.
There’s a fantastic book that discusses keto and cravings. The Case For Keto by Gary Taubes
The Case for Keto by Gary Taubes is a great book that discusses how keto and low carb diets work great for weight loss, diabetes, etc.