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.


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See: [GENERATION AND TESTING OF RANDOM NUMBERS
 FOR CRYPTOGRAPHIC APPLICATIONS][1]


  [1]: https://acad.ro/sectii2002/proceedings/doc2012-4/11-Suciu.pdf