Your incorrect assumption here is that determinism comes with no tradeoffs. There are a few outsider analyses on the topic, for example, this one on sparse MoEs [1]. If OpenAI uses sparse MoE as described, then determinism would be possible but inefficient.
Even if it's not sparse MoE, chances are high that the non-determinism is introduced somewhere purely as a performance optimization. The article speculates that OpenAI knows this well and hides it to protect the model internals.
[1] https://152334h.github.io/blog/non-determinism-in-gpt-4/