Lighter and thinner is NOT fashion over function. It is function in itself. Lighter, thinner also means goes more places, and is more practical to use in many scenarios.
This isn't about identical twins, or marketing. What it is about what people have been wanting for many years. Also very few phones bend, but most phones are thin and light.
Lighter and thinner have their place when it comes to selling Navy SEALs tactical hardware in titanium instead of steel. In those situations, you're not having to compromise the strength and reliability of the hardware.
They do not have as much of a place when it comes to something as fragile as a laptop, especially when us humans got rather used to companies like IBM making a serious fucking piece of hardware that was damn near unbreakable.
It didn't get that reputation because it looked like the thin-ass shit we have today that can break by looking at it wrong. And in our disposable society, we sure as hell can't count on vendors to give a shit anymore about hardware longevity. Their answer every time is to sell you more hardware.