"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.