Because it's not possible. There is nothing you can say to the LLM that will guarantee that something happens. It's not how it works. It will maybe be taken into consideration if you're lucky.
But if you're trying to tell me that every time you list criteria you get them all perfectly matched, you're clearly gifted.
I'm being deliberately pedantic, but depending on what kind of representation we use for the neural network (due to rounding) as well as the choice of inference (that is, given a distribution for next token, which one to choose), it can absolutely be reproducible and completely deterministic.
Though chaotic, which I believe is the better word here - a single letter change may result in widely different results.
We just choose to use more random inference rules, because they have better results.