Because everything always takes the path of least resistance. If a particle is staring down path and A and B, it's always going to take the A path, even if it could choose B, how would it be able to? There's no dice rolling in it choosing it and computing it. If it didn't always take the path of least resistance, on the quantum level, we would have freak occurrences. In the quantum world, everything moves with absolute precision. The "random walk" of electrons may appear to random, but based on my reasoning before, I highly doubt it, more or less minuscule resistance changes on a wire, from other electrons, magnetic field fluxes, and other microscopic quantum forces.
I believe there's something different which makes a living thing tick, which is why we can have in my opinion, spontaneous randomness, consciousness, etc. Perhaps I'm wrong, there is no randomness in living things, and we can create perfectly artificial life that is the exact same as us, with a conscious being occupying the creation. Hell, maybe 200 years from now, they can perfectly reconstruct our brain and bring us back from the dead? If there's nothing really special about us "conscious beings", what would happen if your perfectly equivalent brain was cloned and existed at the same time? Would we have a shared consciousness and be seeing out of two sets of eyes in the universe, acting like a multi-core computer? Or would someone else occupy that brain that thinks exactly like you, but some other conscious being is occupying that brain that has your brain. CREEPYYYYYYYYYYYYYYYYYY. Hell, maybe we are just here for the ride looking out like a television tube, and there is no choices by the being made by you being conscious, it is all part of your brain's reasoning?
A person is but the sum of his memories. If you 2 people that are identical clones and one of them is missing a single memory, he wouldn't be the same person. It would sorta be like multiple identity crisis.