>>77
I don't want to be like one of those people that gets really mad and calls you names and shit, but I just want to put it out there that you are wrong, so that other readers not familiar with game engines wont be deluded. You are right about some things, like frogger would be pretty easy to make entirely from scratch. But game engines for non trivial games are very complex. Even the doom game engine an impressive piece of work.
http://www.doomworld.com/idgames/?id=8802
Many games don't implement an engine. They'll either use an existing engine and add only content, or they might take a predecessor engine and hack in the extra needed features. Game engines aren't easy. You will have to get low level to get the performance you need. The algorithms used will need to scale well if the game is to handle large environments and large amounts of entities. Realistic and reliable physics. Work all of this into networking for internet play. Sound and doppler effects. Efficient and good looking graphics for large levels. Culling areas that don't need to be drawn. Realistic and not stupid AI, which must also be fast enough to not drop the framerate. And then there's the art and general media. And the game itself. Even with all this work, you might fail to make something that is fun to play.