I keep seeing a misconception that transmission lines are made out of copper.

For the vast majority of lines (especially overhead), that's not the case.

Most power lines are made from a steel core wrapped with aluminum conductors.
(that's why power lines look silvery and not coppery)

Steel provides the strength, and aluminum provides the conductivity.
(they both provide some of each but that's primarily how it works)

But isn't copper more conductive than aluminum?

For the same cross sectional area, aluminum is only about 60% as conductive as copper, but it's also only 30% as dense as copper.

For the same mass of metal, aluminum is about twice as conductive as copper.

Copper is better when you're space constrained (like in an electric motor), but aluminum is better when you're weight constrained.

Aluminum isn't just a cost saving measure (although it does cost 1/4th as much per kg). For a lot of applications, aluminum is actually better than copper.

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