Not sure about the exact thrust of your question but a few points:

- flood management is not easy to monetize so there is not much incentive for private industry. The timelines for design decisions (100 year, 500 year) often don’t mate well with private incentives

- it crosses many property boundaries which makes it hard to manage unless you have the rights of a government

- much of the work is still done by private companies but managed by the government, just like other infrastructure works like roads, bridges etc.