Do milk and butter not work in place of cream for whipping, and why?
Heavy whipping cream is homogenized as @myklbykl mentions. That means the fat molecules are pretty evenly dispersed throughout the liquid parts, giving you a smooth mixture instead of one that separates.

When you whip cream, you don't just incorporate air. You also agitate those fat molecules and they start sticking together. As long as you keep whipping vigorously, the fat molecules Wil Form a matrix within which the tiny bubbles of air and liquid get trapped, making a foam. If you keep beating, all the fat globs together, and that's where you get butter and traditionally buttermilk.

So if you add butter to the milk, even if you melt it- it's already done. The fat was already beaten and globbed together completely. You won't be able to get that matrix to form again since it's already been completely whipped.

And unfortunately you can't homogenize the fat and liquid back together. It requires special machinery. I actually tried once because I was young and the internet lied to me. End result was me crying over a big bowl of milk with lots of tiny butter chunks in it.