While your suggestion seems reasonable, one problem is that edit summaries aren't editable by normal users. That could make it too easy for a bad actor to drop in spam, or links to porn, or other undesirable content that would be difficult to get rid of without Moderator intervention.
I think a case could be made for links that stay within the Stack Exchange network be made "clickable". Elsewise, we'll need a way to report "bad" links in summaries.