Those same people talk about Apple having best everything. Since when aluminum cases are considered the best? Sony using carbon fiber cases builds ultralite laptops that weight as little (or less) as Macbooks Air but pack more components (for example, two SSDs)> Here is one such model. Other manufacturers prefer magnesium alloy which is also much better case material than aluminum. Admittedly, aluminum is cheaper.
Magnesium compounds can make great housings, for sure, but aluminum has superior thermal characteristics; hence the popularity of aluminum PC cases IE Lian Li, etc.. Carbon fiber is great, but I doubt you'd be able to do the unibody thing.