Except they've done at least two or three episodes that have explicitly shown you why. Most recently with Cade and Slip here in this two parter, but before that with Cody. Because the clones know right from wrong and won't all just follow orders blindly even if that's how they were raised/trained. Too much Jedi influence, most likely. Cade fired on Kamino. On his own home. And that act tore him apart inside until he had to stand up and try to expose the evil he'd witnessed.
Can't be having that if you're trying to run an evil Empire. Stormtroopers can be more carefully molded in the culture the Empire wants to indoctrinate them within. And because they are more individualistic in nature they can be turned against one another much more freely. Which answers the question of "Why not get rid of current clones but then just build an Imperial clone army." The Empire wants a bunch of faceless yes men, who will do what they are told without question and will turn on each other at the drop of a hat if one steps out of line. That's a hard view to foster amongst the ranks of clones, who are by their very nature a kind of family.