They're actually not distributed evenly, so it is possible to hunt and find them with a reasonably high probability of success. This has probably been discovered multiple times by people independently, but was most widely publicized by Flowing Data two years ago. I don't know if the producers ever explained why Daily Doubles are more likely to appear in certain places. Perhaps they intend for observant contestants to be able to guess at their locations.

http://flowingdata.com/2015/03/03/where-to-find-jeopardy-dai...