Because they literally can't. The factors discussed in that link prevent the Corp from doing that, even if the math says different. May sound stupid, but it's the truth.
But in the Corps defense, they have done something like that... back in 2010 or 2011 (forget)? The Corp released water from the Missouri river reservoirs, in order to make room for the spring meltwater. We all remember what happened then... the Missouri flooded major areas of NE, IA and MO. I-29 north and south of Council Bluffs, IA was closed for weeks. The gov't paid out a ton of money for that issue, both in repairs to roads and levees, and in damages to farmers.