I would say: because a totally deterministic, nonstochastic universe becomes subject to paradox theorems about Laplace's Demon.[1] You need at least a little bit of stochasticity to make prediction and unpredictability work out.
Most of the disproofs here don't actually seem to disprove the demon's existence, just limit it's ability to answer trick questions.
"Suppose that there is a device that can predict the future. Ask that device what you will do in the evening. Without loss of generality, consider that there are only two options: (1) watch TV or (2) listen to the radio. After the device gives a response, for example, (1) watch TV, you instead listen to the radio on purpose. The device would, therefore, be wrong. No matter what the device says, we are free to choose the other option. This implies that Laplace’s demon cannot exist."
This doesn't prove that the demon can't exist, just that it can't answer the question. If it knows what you will do in any case of its response, and it knows what its response will be, it knows what you will do, generally. It just can't tell you what you are going to do. Sort of a difference between being omniscient versus being omnipotent.
In other words, the demon knows that its response will be X, and your actions will be Y, it's response just can't be truthful. It's all still perfectly deterministic.
Free will is just a convenient illusion based on the fact that predicting the universe faster than realtime would almost certainly require more resources than the universe contains. At best, we can limit our scope for imperfect predictions based on imperfect knowledge and limited processing ability. The universe is likely deterministic, but there's no way to act on that in a meaningful way, so for the purposes of human existence, we may as well act as if we have free will.
Free will is an outcome of a universe which is not purely deterministic if you have a proof that the universe is then essentially there is no freewill.
Freewill goes beyond the concept of your brain deciding what to have for dinner.
People are actually analogous to some physical system it's much harder to predict the actions of an individual whilst predicting group actions on a larger scale is easier since the various individual inputs are effectively canceled out.
This is like modeling say a glass of water, modeling each individual molecule is nearly impossible because you get to the point of not being able to measure them especially when you trying to measure or predict the sub atomic make up of each molecule, but modeling the entire system is easy.
Free will can exist in the same way God can exist. At least with our current understanding of the universe, we lack a falsifiable test to disprove either. However, there is also no evidence to support the existence of either with any greater certainty than the claim that there is a teapot orbiting the sun somewhere between the orbits of Earth and Mars.
You are welcome to believe in God, orbital teapots, and/or free will, of course, so long as your belief doesn't lead to actions which create a negative imposition upon others. Sadly, that is all too common, and far less welcome, in my eyes.
> You need at least a little bit of stochasticity to make prediction and unpredictability work out.
There is already unpredictability in all sufficiently complex formal systems. For instance, it's unpredictable whether Turing machine T(i) will halt. Determinism is all you need!
[1] -- https://rjlipton.wordpress.com/2014/08/08/laplaces-demon/