There are number of reasons in which you have multilayer boards, and when it comes to high speed design, DDR3 for instance, there is alot more happening than just the connections from pin to pin.
At high speeds, the physics behind electric, and magnetic feilds become a factor as well as power speed requirements. It's no longer a case of just connecting from point A to point B. The route you take, will have an effect, so at high frequency, you might actually lose space because you cant/shouldnt route signals in this area, or near this group of signals etc.. Power supplies are slow, and cannot keep up with the demand of current in digital circuits. You could have a power supply right next to the pin, and your chip may still not work well, because digital circuits require fast currents, and lots of it. The power supply might have a high current rating, but a power supply does not have a fast response. And thats where decoupling capacitors, bulk capacitors, and the overall power network distribution come into play. All these things are required for high speed, and some of them depend on the layer stack. Not just the number of layers, but what the layers actually are.
Controlling the feilds, and reducing their effects, EMI, sheilding, inter plane capacitance, signal integrity, power integrity and routing complexity, are the reason why you have might have a multilayer board vs a 2 layer board. You could MAYBE get away with a 2 layer board, but you would have to either model the circuit board (parasitics) and depending on what your high frequency content is, have look at and see if all your requirements are being met.
So can you reduce the number of layers ?
Yes you can.
Will it work ?
Yes. No. Maybe. All of the above.
Try searching this site, for some of the terms in bold. It might answer some questions, or create some new ones.