There are reasons to give key stakeholders the opportunity to officially sign off on a new software release. We need some formal approval from the people who commissioned it, or at least their delegates. This last stage prior to release is commonly called the user-acceptance test and executed in a UAT environment. It’s an indispensable stage, but treating it as the final step in testing is problematic for several reasons.
Let me start with a car example. Dealerships are generous with free test drives for the same reason that clothing stores let your try on three different shirts. It’s to let the endowment effect do its dirty work: wearing (or driving) something feels more like you already own it. It's to get a taste for the look and feel, and a catalyst for closing the deal. It's not about really testing the vehicle -- they expect it back unscratched. Toyota doesn’t need their customers taking an active part in their QA process.