The same reason \\ does not autocomplete to all the computers on the network: Because it would destroy corporate networks.
Showing folder sizes "all the time" means that when you open, say, the root of a large server, Explorer would start running around recursively enumerating every single directory on the server in order to compute the folder sizes. One person doing this to a server is bad enough. Imagine if hundreds of people did it simultaneously: The server would be hammered continously.
Even worse: imagine doing this across a limited-bandwidth link like a VPN or an overseas link. The link would be saturated with file enumerations and wouldn't have any bandwidth remaining for "real work". Even the change-notifications that Explorer registers are cause for much hair-pulling on corporate networks. (And these are change-notifications, which are passive.)
Even on a home computer, computing folder sizes automatically is is still not a good idea. How would you like it if opening a folder caused Explorer to start churning your disk computing all the folder sizes recursively? (Then again, maybe you don't mind, in which case, go nuts.)
(Of course, the question sidesteps the question the linked article tries to address, namely, "What do you mean by the size of a directory anyway?")