I don't think we can make a touch-keyboard because it would probably have to be different for every game. It's also a difficult technical challenge to make it so I would simply prefer game developers properly supported touch controls, which is a more natural way to control games than a virtual keyboard.
As 
Kyatric notes, Mouse and Touch are two very different forms of input, both in terms of what they can do, and in accuracy. So it is much better for game developers to treat each separately. Also people tend to make games dependent on features like "Mouse is over object" which will never fire in the touch version of the game, and it's all too easy to miss that if you design your game on a desktop. Using a separate touch object forces you to design your game to the capabilities of the input method.
FYI you can switch on 'Use mouse input' for the Touch object and clicks will count as single touch input. This is great for testing simple games, but obviously a mouse cannot simulate multi-touch. The Touch object does also fully support multi-touch, the only gotcha is the Touch.X and Touch.Y expressions only return a single value, but usually you don't need them. Basically, there's no shortcut to having a real touchscreen device, using the Touch object, and using Preview-on-LAN to get your touch input working really nice... but not everyone has a touchscreen and wifi, which is probably the main reason lots of games don't support touch.