You and I may not agree on much, but I was about to point the same thing out
Epcot's tunnel starts right beside Energy, links to the Innoventions/Communicore building where Guest Relations is located currently (that's an important distinction, since Guest Relations wasn't always there but the tunnel was) and then goes under the plaza and over to roughly the same spot over on the other Innoventions/Communicore building.
It really only exists to let the "central" buildings be accessed without having to go through the park. The east buildings come from the perimeter and the west buidlings come from that side's perimeter.
And operationally, Epcot is lopsided. All the Cast flows in/out from the East behind Test Track/Mexico.
Edit: I feel like I didn't explain that very well.. As far as costume rules and theme are concerned - Epcot is treated a bit different than the other parks. Epcot Future World CMs can go anywhere in a Future World "common area" - essentially outside of a building. They aren't "land locked" like other costumed cast are in other parks, who technically aren't supposed to be cutting through other themed lands. They are technically all the same land. In World Showcase it's the same idea with the Promenade - it's open to all the World Showcase CMs to go to/from but they don't go into the wrong pavilion.
There's actually a group of costumes that's considered "Epcot neutral" that can go between Future World and World Showcase. Otherwise the two groups almost never mingle.
The only park that's 100% "land locked" for costumes is MK. HS, AK, and EPCOT all share the common area as you mentioned. At AK you'll regularly see CMs go through the main gates in costume with all the guests.