The thinking is that most Shadowrunners don't have the facilities or teh resources to produce severalprototypes, then an eventual production run, of a brand new from nothing Thing, be it a gun, a drone, or, especially, a vehicle.
Not to say that those rules won't exist, but they're far more likely to get a PDF of their own than to take up space that could be dedicated to stuff more likely to be used by a larger swath of the audience.