Why was a 2D entry point system, where we sit there waiting for our turn to jump without shields ("please grief me"), chosen for jump point access instead of a 3D sphere several million kilometers wide where people can enter from all directions?

I think your analogy right there, holding that up, is the perfect way, because that's literally the mechanic of it. You've got your tunnel, your jump point tunnel, and you can't see it, but as soon as that opens up, that's your entry. You can't enter from over here or over here. You have to go through the doorway.

But it's actually a physicalized space.

Yeah, exactly.

So that's why people have to enter from one specific direction, because it's a physicalized thing. It's a physical doorway.