Because even the best experts are not going to be able to identify requirements the users won't discover until they begin using the product.

You can approximate some of this feedback by observing user interaction with prototypes, but if you pay attention, you'll learn more when they start using it in production.

In my experience as a designer that changes very little - In reality it can often mean you did a whole lot of upfront work just to have it change anyway. You can put 100 UX experts in the product design phase and users will still have complaints the second you release the product.

The point of this process is to start that complaint processes as early as possible so you're not delaying the inevitable or spinning your wheels on something users don't want anyway.