Holy cow, I wrote a book!
"Why start up another program to see folder sizes,
when they should just be right there, in Explorer, all the time?"
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?
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