Do milk and butter not work in place of cream for whipping, and why? I’ve seen claims that, while you can replace heavy cream with the proper proportions of milk and butter in recipes that don’t require whipping, the mixture will not form stiff peaks when whipped, even if it contains the same milkfat percentage as heavy whipping cream (>35%). However, offhand I don’t know why this would be so, and I can’t find an explanation of the claim.

I’ve used milk and butter to stand in perfectly well for cream in things like sauces or drinks, but I’ve never tried whipping it. If it’s not possible, why? Does it have something to do with the proportion of protein, fat, and water? Does the process of churning butter break down protein/fat structures that are required for holding peaks? Or is it not impossible, but simply difficult or time-consuming to get the butter to emulsify properly back into the milk and cool to the point that it can hold aeration?
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.