Except they have to get all of those millions of lawsuits dismissed individually by advancing that argument and failing in even a single of those cases through a novel argument on the part of plaintiff, sympathetic judge, etc could open an avalanche of lawsuits against the firms.
It also would force the courts dismissing the lawsuits to grapple with the sheer scale of criminality that the dismissals are being used to hide -- it's easy to hide it when you shut down a handful of lawsuits and the vast majority of your victims remain silent, but are judges going to be so accepting of mandatory arbitration when they've already, personally, dismissed thousands of suits because of arbitration clauses that are about the exact same kind of fraud perpetrated by the exact same people and have thousands more left on their docket? How many times do you believe that a judge can see the exact same crime happen from the same perpetrator and just go "Ho, hum, nothing to see here!" Sometimes, the key is just to be really noisy and annoying so they can't ignore the issue.
The act of filing the suits, particularly in a concerted bulk effort, and forcing the government to address and dismiss each individual suit is an effective form of speech and protest.
Your argument is shallow excuses for defeatism, which in turn excuses not actually trying.
Edit:
It also introduces a novel argument of "Uh, so this court has dismissed over 10,000 lawsuits about this exact issue -- clearly we're being abused by a corporation and petitioning the state for assistance. [Argument why the state is obligated to assist.]"
It forces the court to explicitly address the argument that the arbitration system is being coupled with a power imbalance to enable criminal action against entire states of people by documenting the scale via dismissals -- which are public record and can be used as evidence in subsequent cases.