Well, one of teh main considerations is, as Ricador said, money and time. Another would be (and this I presume) long loading times. Finally, you have to remember that a lot of games are being submitted by outsiders. Therefore, you can't really go and tell them, "Make an FPS". That's just the way things are, the way I see it. Things are fine the way they are.
well there is many banners of Combat Arms here on AG.
Because it takes a lot to make one of those
yup. And you need a different knowledge then "flash". Because fps games don't use flash. Anyways when people come here they want to play flash games, not fps games.