They make jet turbines out of aluminum, too, so what's your point?
We are not talking about racing cars or jet turbines. We are talking about laptops. Typically laptops don't use the highest grades of carbon fibre, or the highest grades of aluminum for that matter.
I'm glad you mentioned this. Carbon bikes are valued for their lightness, but they are not typically known for their resistance to impact, with real world designs. For the grades and designs of carbon fibre typically used for mass produced bicycles, they have a tendency for catastrophic failure upon significant impact. For this reason, while carbon fibre is really popular for road bikes, for downhill bikes, not so much.
For a metal, aluminum tends to be brittle, but it's less prone to cracking than carbon as implemented in bikes. With the same sort of stress, aluminum will often deform (yet still can be ridden), but carbon will just crack.
You can successfully design a mountain bike to use carbon, but you have to design around carbon's limitations. The problem with this is increasing weight vs the lightest carbon fibre designs, cost of materials, and just as importantly, the lack of experience by some designers. Do you really think that most laptops built with carbon fibre have the same design experience behind them as those who design carbon bike frames or racing car parts?
That's why it's not uncommon to see carbon fibre laptops with cracks and what not, some even with just regular use and no impacts.
Magnesium is used partially because it is generally cheaper than aluminum when you consider overall manufacturing costs. The material costs are not super high, and the machining costs etc. are not as high either. Aluminum material costs and production costs for the parts are generally more expensive, or sometimes about the same.
Both magnesium and aluminum are "premium" metals for designing laptops. The each have their pluses and minuses, but all things being equal, you're actually often saving money (lower production costs) by going with magnesium, which is one reason why companies choose magnesium over aluminum.
BTW, Apple has already been using magnesium for a long time. The old aluminum G4 PowerBooks weren't made from just aluminum. They were aluminum and magnesium. Even the polycarbonate iBooks had magnesium frames.
P.S. I love steel, but it's heavy. For phones I think steel is fine though, since I kind of like the extra weight actually, for a phone. For a laptop? Not so much. My next laptop is going to be a 2 lb MacBook. Would I buy a carbon fibre MacBook? Maybe, but if I did, I'd be sure to put a cover on it. With an aluminum MacBook, I probably won't (based on my experience with other aluminum Apple products). That alone would negate the weight advantage in my case.
Having had several Carbon fiber Thinkpads and a few MacBooks the Thinkpads really don't scratch at all. I've never used a case for any of them.
I just view them as workhorses. I guess starting with Thinkpads trained me to just use the things and not worry about cosmetic damage. The Apple machines do very well but they scratch easily so in hindsight it was the Apple machines that needed something to maintain the finish.
By the time I went with the MacBook they were going to be sofa queens anyway and they never left the house. It was actually how I justified the MacBook. They were not going to taken out or carried or used for any work purpose they were just browsing machines on the couch. If I wanted something to take out I would have gone with something else. That's also why they held up so well.
Carbon fiber can come in glossy and matte. These machines mostly go with matte. Secondly I think the material used on the Vaio is not Thinkpad grade. It flexed quite a lot and feel it was mostly plastic with a very thin woven Carbon fiber layer in there.
The Retina MacBook is the only MacBook I would consider. I think it's truly a very nice machine at a fair price. I do wish they made it 13.3" and kept the 2lbs weight. I would actually buy it. It's hard to go from 13.3" back to 12" once you've had it. So many under 2lbs 13.3" machines out there now. The only issue coming from a Mac is that the trackpad may make you tear your hair out until you get used to it.