Why do sites like Reddit and Imgur have what look like random strings for unique IDs instead of incrementing numbers?

I just uploaded an image to Imgur and its URL is http://imgur.com/sXoe4 so ... that's not a numeric or hex ID. I can't go to http://imgur.com/sXoe3 or http://imgur.com/sXoe2 etc.

I understand it's probably randomly generated, but there are only five digits, so there's a chance of duplicates. Wouldn't you have to check it didn't already exist, adding one more database call to the process?