It is NOT a simple chain tension device. The position of the guide pulley needs to be moved and controlled. It may not need to move horizontally , but it does need to be positioned properly vertically, and change as you switch gears.

You could separate the guide pulley and the tension device and put the tension device elsewhere, but it is still the same amount of parts and complexity you are just separating them. Remember, the chain tension device needs to be spring loaded and have a wide range of motion in order to take up chain slack. Yes, this is not rocket science, it is basically like any spring loaded single speed or IGH tensioner, but the cage needs to be as long as one on a typical RD. And this is IN ADDITION TO the mechanism needed to control move the guide pulley.

And if this is on a MTB, then the tension device will probably need a clutch.