Is a duplex actually more expensive tho?

Randomly just Google'd and found this link suggesting the opposite:

https://www.turnerandsonhomes.com/blog/cost-to-build-is-a-2-...

As for your second point, that's true. Though, if voting were mandatory I doubt it'd be difficult to pass.

There's something darkly hilarious about the concept of a democracy forcing people to vote.

In desireable locations the land cost is often more than the construction cost, so splitting that land cost across multiple dwellings makes them more affordable. Sure the cost per sq ft is higher when you are putting in three dwellings under one roof than for a McMansion, but you are getting three dwellings that in aggregate should be worth more than one McMansion.

That was the "Anything other than single family only makes sense when land gets really expensive" part.

Are there cities with land that isn't expensive?

In most cities in the US, land costs are not going to be a large part of the costs of building new construction. Materials and labor will be most of it.