It's a matter of texture. In this recipe, we're going for a creamy, yet slightly firm custard-like filling. It's a texture that is distinct from stovetop macaroni and cheese. Many baked macaroni and cheese recipes use a roux to make a cheese sauce on the stovetop, and then transfer the mac and cheese to a dish, add more cheese and/or bread crumbs, and bake it. That's why I do in my Smoked Mac and Cheese with Bacon! In my experience, the texture is different in a roux-based mac and cheese than with a custard mac and cheese. So making our southern baked mac and cheese evaporated milk and egg makes it come out a bit firmer and more casserole-like, but still able to achieve that rich creaminess that we all love! Plus, you won't have to add the extra step of making a roux -- everyone loves an easy southern mac and cheese recipe!