Because the real reason for the Supreme Court's jurisprudence regarding the Federal Arbitration Act over the past 30 years is the reduction of Federal case load (via diversity jurisdiction). To overturn long-settled precedent they had to apply a series of novel rationales that continue to metastasize. Not that the conservatives on the court are complaining as it (probably not coincidentally) has coincided with a conservative political shift that, like their liberal counterparts from earlier eras, see Federal courts generally and SCOTUS in particular as a vehicle for imposing conservative policies on liberal states.
Feels like that introduces more unintended consequences than if they'd just left it up to Congress to patch the loophole the state found.
It's fractal in a way: federal law enjoys unequal power over state law, so why would they see an issue with that same imbalance at smaller scales?