The contamination does not stay put. Black water wicks into drywall, carpet pad, and subfloor and spreads outward, so waiting turns a small affected area into a large one - and turns salvageable materials into ones that must be torn out.
There is also a claim angle. Most policies require you to take prompt, reasonable steps to prevent further damage, so delaying cleanup can both grow the loss and weaken your coverage. Prompt, documented remediation protects your home and your claim.