I guess my general question in those scenarios is why spend time and water maintaining a lawn of plants that can't grow normally in the local climate? There are definitely plants which survive in the area year round in many places in the US.

It's an expectation. Sometimes just peer-pressure, sometimes with legal backing (there are many HOAs that require lawns, sometimes with a specific list of allowed grasses).

I haven't investigated it because I don't have kids, but I think whatever comprises a typical domestic lawn is a better play/recreation surface than any kind of plants that are native out west.

I think the alternative is artificial surfaces which of course have benefits and drawbacks.

I'm hoping to replace my lawn with low/no-water alternatives soon, but, hey, landscaping is expensive.