In Oakland? Most of the homes in the region were built in the 1920s-1930s, though some were newer developments that were only about 10-30 years old.

Wooden construction is very common in America because wood is very common and hence cheap. In this particular region there's another reason for its use: it's right on top of a fault line, and wood-frame residential construction performs very well in an earthquake, while brick or unreinforced masonry tends to crumble.