No, it isn't a lack of ideas, it is a lack of desire.
I'm in college for game design and many of my teachers worked in the industry for years. One thing I hear all the time is this:
People submit brand new ideas on a daily basis in a game company, but not even 1/10th of a percent become games. This is because, since games are so expensive now, many of the big shots in power don't want to take the risk. It has nothing to do with people not having original ideas, and everything to do with the fact that publishers rather fall back on something that did well over potentially wasting money on a new IP that won't sell. This is common sense, really, and a lot of people will say the same thing. It is easier and financially safer to make a sequel or spin-off for a popular series than to make a brand new one.
In any given year, designers around the world submit hundreds, even thousands of original ideas. It isn't a lack of originality, it is the fact that people don't like taking risks when you are talking about something that costs millions of dollars and years to make. For all the bitching and griping we gamers do on this particular subject, the fact of the matter is that we often don't check out a new IP. How many of you guys bought Fallout 3 over Valkyria Chronicles? Or Red Alert 3 over EndWar? Or perhaps pre-ordered Prince of Persia over Mirror's Edge? I'm not saying that there is anything wrong with this, nor am I blaming you, just proving my point that a well-established franchise usually sells more copies over a new IP in a similar genre.
Besides, there isn't anything particularly bad about sequels. They offer you more of what you liked last year, or the year before. Hell, a really good sequel changes things up enough that you don't feel like you are repurchasing the same game. You can have originality in a sequel, if developers work hard enough to do it. But then we run into another issue: Change too much, and people won't like how far it deviates from the original. Change too little, and people will ***** that they were ripped off. But that is a discussion for another time.
