I'd argue it is mostly a technical constraint in some LLMs which is due to a few optimization factor (injected temperature, random rounding error caused by parallelism). In practice you could very well create a LLM that always reply the same thing for the same input, but it would take more time to complete (to be sure that the operations are made in the same order). I don't think those ones would differ so much from the "random ones" to call the firsts conscious and the second non-conscious.