Not to mention that AAA games take 2-3 years to produce, whereas Steam Linux has been out for only 6 months. There's a lot of inertia to overcome.
From a technical standpoint, Wine is extended to make the game work, that version of Wine is bundled with the application, and a script is included to run it without further user involvement.
I don't get why people still insist this sort of port somehow doesn't count. If it runs at full speed, shows up as a "Linux game" under Steam, and the user literally can't tell the difference, what does it matter?