> Why go from something objective/provable to something that does "strange things", when we already have former?

Because we like computers to feel like they are fast, mainly. Most compilers, for example, are non-deterministic because they can be made to run faster if they ignore things like thread execution order. Same goes for LLMs. Technically they are as deterministic as any other software, but we allow GPUs to play fast and loose with floating point numbers to speed things up, which gives the impression of "strange things".