Flash games exist in a strange world. Anyone can make them. They're not bound by big budgets or proven formulas like their console cousins. Any motivated programmer or 14-year-old kid can make a game and get it featured on Newgrounds if it's good enough. Any time someone has a novel gameplay idea, everyone else wants to put their spin on it. As a result, some weird genres emerged. XBox games come in clear, simple genres like "sports" and "shooters," but flash genres are more along the lines of "games where you fling things with your mouse." These are the most popular game genres you can play in your Web browser.
All motorcycle physics games can be traced back to the old PC game Action Supercross (later Elastomania). It was a motocross game where you drove an elastic motorcycle through twisty levels, collecting apples and trying to reach the goal. Flash developers decided that elastic motorcycles and collecting apples were too much for the fickle Internet audience. They stripped it down to the basics. As a result there are around a thousand games where you reach the goal by pressing back to lean back, forward to lean forward, up to drive, and down to brake. Originality aside, there are a surprising amount of good motorcycle physics games. They're all exactly the same, but if you pretend each new game is a level pack you can play forever. They're not specific to motorcycles, either. Monster trucks, skateboards, bicycles and Mars rovers have all proven that they can lean, accelerate and brake.