Titanium is strong, but is more flexible than most materials. They make guns out of it but need steel barrels b/c Ti is too flexible. Everyone has some romantic idea of what titanium is, its not some ultra futuristic material, its not as great as you think. Why not a ceramic composite? Like body armor lol. Or like the ceramic on the space shuttle that is uber efficient at heat transfer.
probably uses alum due to the cheap cost and when you drop your uMB/MBP it dents = accidental damage not covered by applecare...