Being truly random is necessary in certain situations; for example during part of a key exchange protocol, where a permanent master key is used to encrypt less expensive session keys to send over.

If the process of key generation were to be "deterministic", an adversary could otherwise guess the next session key that we'll use, or perhaps even manipulate the user into selecting a session key of their choosing.

See:  GENERATION AND TESTING OF RANDOM NUMBERS FOR CRYPTOGRAPHIC APPLICATIONS

Being truly random is necessary in certain situations; for example during part of a key exchange protocol, where a permanent master key is used to encrypt less expensive session keys to send over.

If the process of key generation were to be "deterministic", an adversary could otherwise guess the next session key that we'll use, or perhaps even manipulate the user into selecting a session key of their choosing.

EDIT: As an aside, take pseudorandom generation. All that is required in order to discover and reproduce a pseudorandom sequence is the algorithm used to generate it and the initial seed. Therefore, the entire sequence of numbers is only as powerful as the 'randomly' chosen parts - but a given seed will always determine the same pseudorandom number!

See:

Random V Pseudorandom

GENERATION AND TESTING OF RANDOM NUMBERS FOR CRYPTOGRAPHIC APPLICATIONS