Obviously they could, but that ship has sailed a long time ago. Apple might eventually reintroduce an upgradeable Mac Pro (they hinted to it, I really hope they do), but for laptops I don't think it's ever going to happen again.
And this is not limited to Apple either, pretty much all the market and makers are going this way. I miss my upgradeable 17" MBP, but I've long come to grips with the idea that I'll never own another laptop like that again.
Simple answer is yes but it would be a radical change of materials and design for Apple
Simplistically the Ali case we are familiar with is machined from a solid leaving a relatively open interior to fit components etc, they play around with materials and thickness of the case left to give the rigidity, hence some of the little ridges you see. Introducing additional supports would mean the case could be thinner yet remain ridged but would hinder the relative openness to place components, but the saved weight could mean the case could be deeper
Opting to go for cast and machined magnesium alloy (which has it's own set of problems) would probably give a greater scope to achieve a deeper case with the same weight and rigidity as far more complex shapes and supports could be cast that would be almost exhaustive to do by just machining (See the surface pro or book vents) of course same issue on openness for placing components
Some of the carbon fibre/composite designs have already shown that it's possible eg some of the top of the range Sony Vaio Z laptops component for component were actually lighter than the MBA's but were not as thin, but thinness got the headline however the CF designs tend not as ridged generally and are designed to flex
Material and manufacture methods are extremely complex with a series of trade-off that has to be balanced.
For volume production and finish coating/colour options Aluminium Alloys are probably the better/cheapest option overall compared to Magnesium and Carbon composites that may offer other benefits
I quite like the weight of the MBP - it gives it a nice feeling when using it. Using something like a rMB is a very different feel and while I like it when out and about, I much prefer the heft of a MBP when working at my desk.