Silly question, but I’ll ask it anyway.

Wouldn’t work for interconnects. There is only one conductor covered by a very high voltage rated insulation. You need two conductors for an interconnect. A supply and a return.

Yes but the real MacGyver would know this and use two spark plug wires. Connected to a distributor so you could switch components really fast. Not only that but supply and return? The real MacGyver would know that’s plumbing not electrical. You’re not even on the right aisle.

Just wondering, but can you make an interconnect with spark plug wire?

Why yes, MacGyver, yes you can. But it would be a lot more impressive to make one with pencil and paper. Graphite is conductive, you know. 

Oh wait! I know! What about Wrigley's chewing gum wrappers? Hey, this is fun!

why is or isn’t this done?

Uh, sound quality?