What you want is to bypass steam and that won't happen, nor will it help, and here's why.
Steam is not just a distribution platform, it's also a DRM system. So yes, Steam does allow you to download your games to your computer. But every time you start your game, Steam servers authenticate your local copy as authorized. Games distributed on CD/DVD will often use those media to authenticate the copy, games that are on steam use Steam and are specifically coded for that. So even if you start the game's exe directly, it will expect Steam to be running. Even further, if you look at shortcuts created for specific games, you will see that they actually start steam first. So going around steam would in effect require something that is usually illegal in most countries, like a cracked executable.
So unless you have a DVD for your games, you have to go through Steam, period.
It won't help to circumvent Steam anyway. I have used steam on Windows and now on Linux (native and Windows versions) and have never had a graphical problem related to Steam. The problem with graphics always comes from somewhere else. In your case, I would point fingers at your Intel video card.
Most sites list the bare minimum as Geforce 6600 or higher or Radeon X1600-Series for Tropico 4. I don't believe your Intel card matches that. Intel has made great strides in video card development, but they have yet to produce a video card truly capable of running games. Depending on the game, your mileage may vary. You machine is a beautiful beast, but has been built with a workstation video card, not a gaming video card. If you really need this running, check if there isn't some update to your video card's driver, as that might help. Knowing a bit about the Mac ecosystem, I doubt there is a better driver out there, but it doesn't hurt to check.
Unfortunatly, if you plan to play games, you should stay away from Intel video cards at this time. I imagine you didn't have much choice in the matter for a MacBook Air.