Why don't frame_ids use ROS namespaces?

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There is already a hierarchy defined by the tf graph in the child/parent relationships, vs. ROS names where the hierarchy is defined by the name.

namespaces are used to determine how communication channels are connected. Whereas tf frame_ids are related to how coordinate frames are physically connected. Thus if you push down a set of nodes relating to the 2d navigation they will work in their own namespace and not have collisions on topics or services. However, the base is still connected to the body in the same way, despite the namespace push down. Thus pushing down doesn't make sense.

There are a few other considerations, tf frames are not necessarily connected to any specific node. For example on the pr2 the base_laser frame is published by the robot_state_publisher based on the URDF, but the hokuyo_node publishes scans in the base_laser frame. And then rviz transforms the data into some arbitrary frame for viewing. If any of them disagree about the full name of "base_laser" this will not work, for the tree in tf will not be connected anymore.