These discs are stamped, and there is DATA there that can't be written by a retail writer.

Do you know how Vinyl Records are made?  They are physically pressed into shape.  The pits of a CD are pressed.  A burner uses a laser to burn ink forming these pits.  The pressed ones can be put in places a burner can't write.  This might help:

Wikipedia wrote:

BD-ROM Mark is a small amount of cryptographic data that is stored separately from normal Blu-ray Disc data. Bit-by-bit copies that do not replicate the BD-ROM Mark are impossible to decode. A specially licensed piece of hardware is required to insert the ROM-mark into the media during replication. Through licensing of the special hardware element, the BDA believes that it can eliminate the possibility of mass producing BD-ROMs without authorization.

It would take forever if they burned games in a burner making it take longer to make a game, there would be less copies of the game, cost for the game would go up, and you would be able to erase it.  I'm sure they would still find a way to keep you from copying it.