The fact arbitration is a standard clause in every contract or terms of service proves it benefits nobody but the business. It's all about making consumers renounce their right to proper legal recourse. Why can corporations make people renounce their rights to begin with? Why even give people rights when every company will put a clause in their contracts that says "oh those are nice but please leave them at the door before doing business with us" ? It makes no sense.
Well, anyone can ask you to renounce certain rights, and you can even agree, and courts can (and for certain rights have) rule that you cannot waive those rights. Even if there's a right that is well known you cannot renounced, you can be asked to renounce it, and you can agree to do so, and still not lose the right.
Arbitration can't completely remove access to the courts -- it can (and does) make it harder to get there. Case law for this sort of thing is always evolving, and so is statutory law. It's entirely possible for the courts to modify arbitration sufficiently to make the playing field more even.
It's because the USian view has come to see rights as axiomatic primitives to be layered on top of rather than context-free qualitative assertions. This is regressive due to complexity-induced contradiction, but here we are. It's similar to how with Turing completeness you can implement a low level interpreter on top of a high level language, eschewing the high level language's fancy constructs.
Part of the popularity is herd mentality. I know arbitration lawyers who find it bizarre - many company’s that would be better served by not using arbitration want it anyway.
