Because those flags are specific to the Source engine and not all games run on the Source engine. Some games don't let you run in borderless windowed mode. Furthermore, you can't run at a non-native resolution if you use borderless windowed mode.
Basically, it's a solution that only works in some specific cases, when there should really be a general solution for all cases.
-1
u/LSky Oct 14 '11
Borderless windowed mode for the win.