Because showing the folder size requires scanning all the files in it, as well as all the files in all it’s subfolders. For some folders that could mean tens of thousands of files. Which would take quite some time.

There are ways to speed that up. But you’d be scanning all the files, all the time to keep a database updated.

Basically, it’s a lot of overhead

Not an expert but I thought I would share my thoughts based on experience. When you look at the size of a file it’s stored in actual metadata of the file so it can be accessed quickly however since a folder is a collection of files it can’t easily keep that info. Once you ask the is what the size it is has to go and add up the values of each file contained in it. This can take a bit of time especially with larger folders so if windows constantly calculated the size of every folder it would just waste resources and make everything painfully slow. Again not an expert this is just how I think it happens.