The reason they don't do this is because they don't want you to spend too much time on the site. If you get notifications of every reply you come back for ever

The reason they don't do this is because they don't want you to spend too much time on the site. If you get notifications of every reply you come back for every one instead of just coming back and checking now and then. Which in turn stops the person you're replying to from coming back to often since your replies will be much slower.