First, cultural: they don't like confronting officials until a crisis has edged into catastrophe. Instead, they wait for the government to take action, and wait and wait, and fume in private.
The other reason is that the combination of miles of asphalt and clogged drains creates the impression that far more rain falls when a storm strikes than actually does. Residents see the spectacular floods that transform large parts of the city into rivers within minutes, and assume with that much rainfall the reservoirs must be filling up.
Man, these droughts -- and energetic reporters reporting on them -- are turning up a lesson a day.
Wonder how many residents in other cities around the world have been fooled by the same optical illusion?