You make it sound like thats easy. For starters the real body doesn't even travel with the others so how would he find him. Nagato has probably sealed himself away somewhere like he had those bodies locked away when we were first introduced to Pain. Finding him is going to be extremely difficult and even if they do figure out where he is, he is a master of sealing jutsus so getting to him without him letting you do it would be close to impossible. Finally there is this, Jiraiya said that Nagato was a genius and mastered all six elements at age 10, so I would say that the real Nagato is stronger than all of his other bodies, plus he still has the rinnegan and we still don't know jack about what that can do.