Yes I understand they need to have ram for the O/S to run, but why make several different models based on storage space internal 16 g, 32g, 64g, and 128g.

Why can they not just make one model and when someone wants more storage space add a SD memory card?
This all we do with a PC is to add another memory card in a slot on the main board and if you want more storage space add a larger hard drive.