Scientifically speaking, why doesn't China build a dam around Bangladesh?

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https://en.wikipedia.org/wiki/Northern_European_Enclosure_Dam

This proposed eu project would take $450 billion, and 75 years to complete

The Bengal dam in my picture would be 1.7x as long, but 1.9x shallower (meaning slightly smaller overall).

1) it would halt the flooding of Bangladesh and India
2) It would create a 712 cubic mile reservoir of water (bigger than Erie and Ontario combined)
3) in one of the rainiest regions on earth (meaning it would desalinate very quickly)