Portals

Here's something that has always vexxed me. I know that Astral Conduits reach from the Prime to the Outer Planes so all the deaders can go on with their afterlives. Fair. They also reach between different layers of an Outer Plane or even two places on two different Outer Planes. Also fair, albeit with a slightly less tangible logic. Now, how about portals? The old Planescape: Boxed Set says the that portals always have one end hanging around Sigil, placing their creation and control squarely in the Lady's hands. But is that to say that no portals reach between Outer Planes? Aren't the major pathways just stock full of them - every branch of Yggdrasil and every cavern of Mt. Olympus? What about the odd paths that let you go jump between layers of the Beastlands just by walking under a branch, or the little doorway at the back of Tiamat's layer that drops a sod on the outskirts of Dis? Are they not portals?

Discuss. =}