You can enable this in Window – Preferences – General – Workspace – Refresh Automatically (called Refresh using native hooks or polling in newer builds)
The only reason I can think why this isn’t enabled by default is performance related.
For example, refreshing source folders automatically might trigger a build of the workspace. Perhaps some people want more control over this.
There is also an article on the Eclipse site regarding auto refresh.
Basically, there is no external trigger that notifies Eclipse of files changed outside the workspace. Rather a background thread is used by Eclipse to monitor file changes that can possibly lead to performance issues with large workspaces.