The problem is not that there is no standard -- there is, it's called ROS. The problem is that manufacturers are not interested in opening their robots up to that degree.

Also, REST is a terrible idea for interacting with robots. REST is call-response based. But with Robots a pub/sub (like ROS or MQTT) or a blackboard data architecture (like what we used in Transitive Robotics) is much more efficient and natural.