I will give you what I believe to be the reason.
It is because of CSS copy protection. For a DVD player to play a movie that has CSS, it must get several keys from the disc. These keys are always stored in the leadin in an area called CPI (Copy Protection Information). In order to keep pirates from being able to make DVD-Rs that have CSS on them, the leadin area is pre-written to all blank DVD-Rs. This means that a DVD-R can never have the CSS keys on it in the correct location for the DVD player.
A DVD burner has to write a new leadin in a different place when you burn the disc. Only a DVD burner knows about this other leadin.
None of this mattered much after CSS was cracked.
The pre-written leadin also has other information such as the PFI (Physical Format Information) and DMI (Disc Manufacturer Information).