Why is their so many sold listing showing as active in Zillow database

Could be 2 reasons:
1) The agent never removed the listing from the data feed.  There are many services that agents use to send listings to us, and it is important that they are removed from these services when the listing sells.  If you scroll to the bottom of any listings, you will see the data source.  Taking it out of there takes it off Zillow.
2) Sometimes a listing comes to us via a feed, but then the agent manually changes the listings, thus breaking the feed relationship.  When this happens the listing is manually managed moving forward - including taking it down.

To remove a Sold listing, just go to the My Zillow tab and click to see the listings.  On the page of listings, each one has the option to Edit or Cancel.  Clicking Cancel will immediately take it down.  However, please be advised to also take it out of the orginately feed (per issue #1) to prevent it from being rewritten to the site.

Zillow provides a marketing service for a fee for a fixed period of time.  It does not pull listings based on sold information from a listing service.  So the listing is going to stand for its term unless the person who placed the listing pulls it manually.  Many agents are probably leaving listings active hoping to flip inquirers to other properties.  Property owners doing FSBO probably just forget about it.  Zillow is an advertising medium, not a listing service.