Your experience is shared by many people in this corner of the continent, where rapid development meets heavy rains. Harboring the enemy Where there's water leakage, there will eventually be mold. Buildings with traditional stucco, brick or other exteriors can and do experience water leakage. Seattle has a much smaller one, but a substantial one nonetheless. The answer begins with understanding the types of building materials commonly used in construction.
Gypsum wallboard is present on the interior side of most residential construction.
Some EIFS construction systems also use gypsum wallboard in a semi-waterproofed state, to provide external sheathing as well as interior sheathing. Such systems are even more vulnerable to moisture and mold problems. It's the paper on the surface of the wallboard that is the problem. Most fungi are unable to grow well in the alkaline medium created by the gypsum. With only paper as food, certain filamentous fungi such as penicillium aurantiogriseum, aspergillus versicolor, stachybotrys chartarum, and others not only tolerate alkaline conditions but also out-compete other organisms.