Because they were all crushed when sega lost the fight vs. nintendo. Except some very few pieces which were probably saved by some sega workers back then.
Sure it is the most rare official Mega Drive game (official in terms that it was meant to be officially released) in existence and also sure one of the most thought after games in history of video games. Estimated price for a non-Alexei Patzinov- signed is above the US$15000.