Sony decided on a date for which they could not realistically meet the demand. So faced with either selling out their stock and creating a shortage on the market, or waiting until they could produce enough units where a shortage wouldn't realistically occur they did what every profit inclined company would do and went into sales as soon as humanly possible - factory output numbers be damned.
This is ridiculous. Most people would rather have a shortage in supply than a delayed launch. Any company would do this and not just "profit inclined" companies. We can't expect them to warehouse over 3 million units (current sales with a shortage still). At what point could we expect them to launch? 4 million? 5 million?
And this doesn't simply apply to Sony, Microsoft also wasn't going to sit on a lot of units before launch.