They aren't. BUT, I would love to see something like this happen for all science journals:
1. You post a paper to arXiv (or equivalent for your field).
2. Preliminary review by whoever is interested (and is qualified to publish on arXiv themselves)
3. Interested editors of various journals contact you with a list of proposed in-depth reviewers (or just accept based on quality of existing reviews).
4. Address reviewer questions (to the extent you think valuable)
5. Post update to arXiv.
6. Editor decides to publish or pass (return to step 3).