MacOS doesn't support PRNG reseeding?
I can only guess at Apple's specific reason for this. But a good reason for not allowing user-level reseeding is explained in a post by D.J. Bernstein here.

Consider two secure sources of entropy $x, y$, and a malicious source $z$:


  
  Generate a random $r$.
  Try computing $H(x,y,r)$.
  If $H(x,y,r)$ doesn't start with bits 0000, go back to step 1.
  Output $r$ as $z$.
  


For some crypto applications (i.e. DSA and ECDSA), being able to consistently predict as few as 4 bits of randomness could lead to a complete break (in the case of (EC)DSA, the recovery of the secret key).

In this case, allowing user-level reseeding is precisely what could allow this kind of attack, since it would be the last source added.

Like he says, there doesn't appear to be a good reason to be adding new entropy to the PRNG all the time, since many cryptographic applications (e.g. EdDSA) show it is perfectly possible to deterministically expand 256 bits of quality randomness into an unending stream of secure randomness. Needing to add entropy just indicates your PRNG has been compromised at some point in the past and everything you've done since should be considered utterly broken. Instead, generating high quality entropy once and keeping it safe is a much better idea.

It seems Apple has taken that approach.