As seen in the movie Parasite depicting the stark contrast between rich and poor South Koreans, there is a scene where the entire neighborhood is flooded

My point being, you can cherry pick and see every city gets flooded. The difference with Saigon is flooding occurs throughout the city in critical areas. Government funds and projects exists to address these issues but the whole amount never gets allocated to properly developing the sewage infrastructure. Instead, hotfixes such as building the roads higher. This is a never ending back and forth with civilians building barriers or building their house higher so the water floods back into the streets. That's just one example of lack of infrastructure