Because your pores are dry which makes your skin dry causing you to itch
Dehydration and itching are sometimes the result of problems with your liver, curiously enough. See your primary care for an evaluation.
It is your skin's way of telling you it is dry and dehydrated.
I suggest you look into good quality electrolyte drinks and water supplements and please drink ample spring water (beyond thirst)
1 hour away from meals.
You may also have a deficiency in EFAs (Essential Fatty Acids).
I hope this helps.
Best wishes,
Niko