Because comment threads can get long. And they fill up fast.
All of the realtime updates are "hidden" loads--they just add a "new question/answer" grey bar, or the aforementioned "show comments" doohickey. If they just loaded on their own without requiring a click, they would flood the page. Whatever you're currently viewing would get pushed down, and it would end up hampering productivity.
On SO, if the "new activity" bar was enabled, and it was made to auto-expand(I had a bookmarklet for auto-expand somewhere--I'll post it later once I find/re-make it), the front page would be pretty much a river of posts.
Look, the devs have saved your F5 key from utter destruction (thank you devs =D ). It's not too much to ask for you to click a "load" thingy when you want to load some new stuff--mouse buttons are pretty hardy fellows. ;-)
Edit: For those of you who like getting flooded, Here's a script. Not tested, though. And SO has realtime updates disabled for questions (not comments), so it won't make much of a diff there.