Do you mean adventure as in the storyline or the adventuring off the path content like finding special encounters/events etc.?
If it's that later than I believe it's simply because the content of finding something is the reward in its own, they achievements are there as something for those players who explore a lot to brag about or feel they achieved something.
@Turelus - EU PC Megaserver "Don't count on others for help. In the end each of us is in this alone. The survivors are those who know how to look out for themselves."