Only an organic creature can use the portals, and it requires destroying your body and then reformed.
Ever found it weird that using the gates at the space station hub spurts out blood when you use it? This might be because in this universe, portals cause you to explode and then reformed on the other side. Perhaps the portal was made so that only humans/organic creatures can use it to. The Earth's core is used as an energy source
The way Earth's core is designed, it seems like its being controlled and maintained. And the Guardian was created so that no one can alter the flow, or use it as an suicidal attempt to destroy Earth. Which given the current state of Earth, is not surprising.