iPhone storage

Why can’t iPhone use SD cards because I have seen online on Walmart that you can buy a micro card reader or something like that and you plug it in we’re your charger goes and Walmart is selling them why can’t people who has iPhones use it if you can’t why do they make stuff like that and actually sell them to people I thought about buying one just to see if it would work but I haven’t did it yet but now I’m really really thinking about it