In arbitration, you do not have a judge, so there isn't subpoena power, and discovery is extremely curtailed.
https://www.gsb.stanford.edu/insights/why-binding-arbitratio...
It's simple. The optimum situation from Amazon's point of view is a system that gives almost enough justice to avoid public outcry, and no more. The most outrageous cases should get paid a modest amount, average cases should get paid a tiny amount, and no stretch cases should get paid. Amazon anticipated that by paying for the arbitration process, they'd end up with a set of pro-Amazon arbitrators and that, in itself, would keep the number of cases filed to a minimum. What they didn't anticipate was enough people angry enough to file cases even in a biased system just to mess with the company.