Originally Posted by DMSmillie
Most likely as a guard against unscrupulous types disguising spammy or malicious links. Easiest to have a single rule for all, trusted and unknown users alike - no links where you can't see where the link goes, either from the visible link text or by mousing over the link.
What DMSmilie said. It is, however, perfectly ok to use the Insert Link button to make links
prettier without hiding the url (it's available on mouse-over or by copying the link).