I don't know the reasoning behind it; What does deletion mean for a post? simply mentions the nuts and bolts of it:
Deleted posts are usually not physically deleted (that is, removed from the system); they're just hidden (AKA soft-deleted).
Since this is generic Stack Exchange functionality, you'd probably have to ask on Meta.SE to get the reasoning.
Now you understand that high-rep users can see deleted posts, so take a moment before you post anything that you might not want to stand behind. I don't know how much you're reacting, but consider this: the fact that you deleted the answer is also visible, and should be taken into consideration when viewing the answer. Simply being wrong about an answer is no big deal, and deleting your answer is (IMHO) the correct thing to do.
Random factoid: there are ~256,000 (non-deleted) answers and ~40,000 deleted answers on the U&L site right now.