This is because games only render with one
viewing frustum. There are only a handful of games that actually support more than one. It takes a lot more processing power to render multiple viewing frustums, and 99% of the gamers in the world aren't using more than one screen, so I wouldn't expect regular games to just start supporting it.
There are a couple of modifications to the Quake 1 engine that render multiple frustums. One is called
PanQuake and the other is
Fisheye Quake. PanQuake renders a 360 degree view, but I believe there are settings to change that, so you can render only how much you want. I don't know if similar settings exist in Fisheye Quake, but it's fun to mess around with. Both are open-source and can be modified.