A mega-hit will make money, sure, but all of the losers tend to balance that out. In my long winded way I am now coming to the point. The reasons for clones and sequels is that they reduce the risk of a new game title. Cloning a proven winner means that you aren't guessing if the genre will be popular. Design time will be reduced, perhaps even development time if you can license a game engine.
Sequels take far less development.
Code may be able to be reused, you can market directly to those that bought the first game, development schedules will be much shorter, and so forth. Anything that reduces the risk and reduces the cost is a good thing to companies living on the edge. Take a simple example. You'd think they have it made. It doesn't hurt sure enough.