Because not having a credit rating is the absence of information, not evidence of being rich/responsible enough to never need credit. You're an unknown quantity, it's impossible to establish what a default score should mean, except "we don't know you, so we're gonna err on the side of not lending you money". What you really want is to be able to record evidence of having bought a car with cash, of consistently saving etc with the credit rating system, but that doesn't exist.

The system optimises for what it (probably correctly) perceives as covering the large majority of situations. Sure, sometimes that leads to some curious corner case situations but it's hard an indictment of the system.