Aluminum oxidizes very quickly and forms a brittle insulating sheath (sapphire) around the wire. Once oxidized further oxidation stops. Increasing the gauge or number of strands allows aluminum wires to work as well as copper, except for flexing. When aluminum wire is bent the coating cracks exposing fresh aluminum underneath which further oxidizes and so on.
Plating copper onto aluminum is difficult, requiring several phases, but can be done. A coating may look like copper but may be something else. The coating flexes without exposing the aluminum underneath. This allows the full diameter of the aluminum to remain conducting while being flexible, which is the reason for stranding the wire.
If it is copper, a thin coating may not be enough to permit soldering. As mentioned by Steve, the solder can dissolve.
While the copper coating is thick enough to protect the aluminum from oxidation, it may not be thick enough to prevent the aluminum from interacting with the solder (a speculation).