Even if everything was encrypted (which is not the case) and that encryption could not be downgraded by a mitm (which is not the case - cf starttls), it can still be used to track whereabouts, or disrupt the connectivity or utilise any mitm zero day.

But a good question would be why 5g, and not every other computer chip in any computer and network equipment, which could be backdoored, and I doubt anyone verified the conformity of the chip to the blueprint sent to the factory.