The video game market is especially derivative at the moment, not just of other games, but of popular films. When "Saving Private Ryan" came out, game manufacturers cottoned onto the fact that everyone wanted to re-enact the Omaha beach scene. Games like Medal of Honour and Call of Duty go to some lengths to actually recreate seemingly identical missions to scenes from the movie. Manufacturers duly noted how well MOH:AA and COD did, so triggering a massive slew of WWII themed war games which were all fairly similar in execution.
Sometimes though, it is just coincidence: Note how "Assassin's Creed", "Infamous", and "Prototype" all came out around the same time, yet all feature similar free-running sandbox setups. All three of these games were popular, so expect a slew of sequels and knock offs.
Either way, manufacturers will lean towards games that conform to what is popular at the moment, as this will guarantee big returns. Hence why FPSs are top dog, and will remain so until people will get bored of them (fairly unlikely: there is no thrill greater than murdering someone, so nothing can hope to surpass the carnage of FPSs in bringing excitement to the masses).
Sometimes though, a derivative can be superior to the concept it is copying: "Mafia" is possibly just another GTA sandbox clone, yet I'd argue it is a far superior game to GTA3. (Also note the way in which Mafia emulates classic gangster movies such as "The Goodfella's").