Why is folder size not displayed when viewing in the details mode?

There's something about Windows that has always puzzled me. Let's say you have a directory in which there are some additional directories (folders) and some standalone files. When in that directory you select details for your viewing mode. Right click on a standalone file, select properties and you'll see the file size. Do the same for a folder and you'll see the folder size. This tells you that the information about the size of the file or folder is present. Yet when you select view details, the size of the standalone files is displayed but the size of the folders is not displayed.

The only reason I can imagine Microsoft does this is because they make a bunch of money when people like me get so annoyed with this that we buy software such as folder size (which I use). But I didn't pay one dime for the folder size software, as I got it free from a third party vendor. So the reason can't be because they make a bunch of money selling the folder size software.

Can anyone explain why Microsoft doesn't display the folder size?

Because it would slow your system down. The sizes of individual folders aren't recorded anywhere on your hard drive; therefore, the size of each file in a folder must be added-up every time you want to know the size of that folder. This process can eat up a lot of resources if you're dealing with large folders. Someone at Microsoft probably just said "It's not worth the trouble!". It's not a conspiracy by the Illuminati.