Why does pasta dough need to be kneaded for such a long time?
Kneading aids gluten development, which does facilitate dough stretch-ability.  You do need to let the dough rest, after kneading.  This allows for hydration, but also lets the gluten structure "relax" so that the dough can be stretched. Gluten development also impacts crumb texture.  So, for, say biscuits, you would not want to knead and develop gluten. For pasta and pizza you are looking for a different final texture/chew.