Because that element of randomness introduces the room that allows decisions to go one way or the other
not dependent on the initial conditions.
The question then shifts to 'is the state of the universe at any point in time knowable', and if it isn't then you probably will have vector through which thinking creatures could have free will. And even if the whole string of chance encounters up to there did not exhibit free will the ability to choose an outcome would quickly become a selection criterion in any genetic mix that ends up laying out a nervous system. After all, the ability to react intelligently to an environment is a huge benefit.
In a 'billiard balls' (as you correctly infer, deterministic) universe it doesn't matter whether minds exist or not, the outcome will be identical. Having reasoning ability in a universe like that would be akin to a cruel version of being along for the ride without any ability to influence your environment. And I don't think we need to look very far to determine that our minds have very successfully influenced our environment so there is some evidence that it makes a difference.
Ah! says the proponent of the deterministic universe: but if it were deterministic it would be just the same! Which to me is just as much a cop-out as saying 'God did it'.
Randomness is a thing, entropy is a thing and quantum mechanics operates at a level that our brains are ill equipped to deal with. It appears to me that free will is real, and our whole society is built on that premise.