First, it would be pretty difficult to search every post in the database when a post was deleted to look for the possibilty that some other post had quoted it.
But also, it would be much more disruptive for someone to have the text of their post changed without their knowlege. For example, what if they say "<quote something from Pat> I disagree with what was said here` and Pat deletes the post and then the post becomes “I disagree with what was said here”. It would then make no sense.
Edit: I never seem to notice that things are in the feature category.