Free oxygen combines with other things very readily, so you need a steady source to replenish it. Abiotic sources may not be common/strong enough. It's been a long time since I've kept tabs on that research, however.
For example, the anaerobic bacteria in your gut do not venture out into your other organs because they cannot survive in the presence of so much oxygen. There are of course membranes and such keeping them in check too, but I was surprised to learn that the oxygen gradient played a role.