A brief explanation why…
After dropping out of Harvard in 1983 to work for Microsoft, Gabe Newell spent thirteen years at the software company where he contributed to the development of Windows 1.01, 1.02 and 1.03 as well as the porting of Doom from MS-DOS to Windows 95. Quake and Duke Nukem 3D weren't released until the following year.
At the time, Doom was estimated to be installed on more PCs than Microsoft's new operating system and after considering a buyout of id Software, porting its game made more sense. Microsoft took this promotion as far as having Bill Gates digitally superimposed into the game for a Windows 95 video ad, while Excel 95 had a Doom Easter egg including portraits of the game's programmers.
Newell and fellow employee Mike Harrington left Microsoft to found Valve in 1996 and create Half-Life after being inspired by Michael Abrash, who previously left the company to develop Quake at id Software. Doom was released on Steam in August 2007 and can also be downloaded for free on TechSpot.