Why don't alloys self-galvanically corrode?
They certainly can. Most cases where the alloying compound is more noble this is a hazard. Nickel in Iron is a popular example, you can get corrosion acceleration on the iron by reduction happening on the Ni-surfaces. 

Where the alloying element is less noble you usually have superficial corrosion to begin with but in the end the resulting surface is protected by the main element - which is common for bronze(s). If you just have lumps, then you do not have a bronze and slowly but certainly, nature will reclaim the tin. 

One has to care about the homogeneity of the alloy.