> Isn’t wire also a shared medium?

That's a really tricky question. At some point, the packets are going to share a link with those from other customers, possibly oversubscribed...

Cable internet is clearly a shared medium at both the physical and link layers.

Fiber like GPON is shared physically-ish, but DWDM isolates individual waves so thoroughly that it's hard to think of that as a shared physical medium unless you're thinking about backhoe fade and the like.

With DSL, do you consider the DSLAM?

Etc