It has to do with your field of view, perspective, distances involved and peripheral vision.
In games with short distances to 3d object scenarios, (FPS, driving, etc) where objects are measured in meters or feet from the virtual players view, you can get a much better feeling of speed, especially if they have a wide angle of view.
Many games have been using fakery to simulate the effect of speed, because at the end of the day we are looking at a 2d flat screen with 3d rendered objects with virtually no peripheral vision to speak of. (incidentally this is where surround and eyefinity gaming really pay off, by increasing the effective peripheral vision. I mentioned this in another thread here about surround gaming)
Anyway I digress, as you can imagine, sitting in a cockpit miles above the ground and away from any objects, compared to a running/driving around in FPS/driving where things flash past you within meters, you just can't get the sensation of speed.
And landing, well the nearest objects are still usually 100s of meters away and you don't get a very good view of the ground from within a cockpit when the ground is below you, so sensation of speed is therefore even more limited again!
If you played on a surround/eyefinity setup you would get a better sensation of speed because of the increased field of view and increased peripheral vision.
Also the size of the screen has a lot to do with it too, a 40" screen is going to give you a better sensation of speed compared to a 24". This again has to do with peripheral vision. The more of the game world you can include in the peripheral vision the greater the sensation of speed.
Another thing that would no doubt help is the 3d effect, which I'm yet to experience in a game (because I don't have 3d monitors), but I'm pretty sure looking at a 3d world with 3d vision would also increase the sensation of speed.
So at the end of the day, we are limited by the fact we see in 3d and are viewing a 3d world on 2d screen that is too small to give us an effective peripheral view of the world. It is a game after all!
Read this for a game developers view on this problem>
http://blogs.msdn.com/b/shawnhar/arc...rspective.aspx