last I checked DOOM was ported to windows a long time back (aka DOOM95) so making it work on later versions wouldnt have been too hard.
Arena/DF were both made only for DOS...so it would have to be ported to windows which is where it gets hard.
Also, Steam wouldnt use dosbox, but if they did want to run DOS games(which i doubt as they would prefer to port them), they would use a similar DOS emulation program of their own.
It all boils down to this, why wold bethesda want to spend more resources on a game that is now released for free?