Because I had no idea it was very bad for performance until Brecht reverted it today. In fact I didn’t know it existed.
http://lists.blender.org/pipermail/bf-blender-cvs/2008-September/015600.html
Revert a commit from 5 years ago that added a glFinish call before swapping on Windows. This is really bad for performance and especially showed in the game engine. The purpose was to disable FSAA and fix some UI issue on Geforce 2 graphics cards, hopefully the drivers have been fixed by now? I have no way to test this, so we’ll see if an issue pops up on those older cards, and a lot of code changed since then anyway so it might not even be needed anymore.
(Brecht calls it a performance killer in the opening comment in the code shown)
If this was bad for performance I wonder how much faster the BGE for Windows will run with it reverted, it was initially made to fix some issues for GeForce 2 series cards which Nvidia may have fixed years ago.