I might just be slow, but wouldn't this work the back and chest at the same time? Is there any particular reason why this type of exercise doesn't exist? Why're we neglecting an entire plane of motion?
Whilst I have never seen a machine that specialises in isolating that movement, there probably is somewhere. There are literally hundreds of distinct designs built for various purposes, and for the preferences and biases of the users or designers.
However, the reason we do not see such a machine in a typical gymnasium is probably because there already exist various permutations of cable machine that can train adduction in isolation. The cable cross-over, for example, is a standard feature of any well equipped gymnasium, as are many machines or apparatus for compound movements—pull-down, pull-up, dip, and so on.
We might reasonably question why, then, we occasionally see machines that isolate humeral abduction, since the same rationale can be applied to those. But they are not common, and where they do exist, it is probably the consequence of commercial interests and the disproportionate focus (real or perceived) that gym-goers give to shoulder development.
Pure shoulder adduction is just a pretty mechanically difficult movement to design exercises or machines around, so what you see more often are shoulder extension exercises (i.e. the lat pullover and straight-arm pulldown), or shoulder adduction combined with elbow flexion (i.e. lat pulldown or pull-ups). The muscles used for shoulder extension are almost exactly the same as those used for shoulder adduction.
Arthur Jones, creator of Nautilus Equipment, did invent a shoulder adduction machine which he called the “Behind the Neck Torso” machine, however it never gained the popularity of his lat pullover machine, which utilised shoulder extension instead of adduction.1
Other options for training pure shoulder adduction would be sitting or kneeling under a cable machine and pulling down on the cables, or hanging upside down and doing inverted lateral raises. However I don't think these would offer any unique benefit over lat pulldown or pullover type movements.