For more crucial PII (such as SSN, health data, payment info, etc), vendors are generally required to have certifications from a third-party auditor (such as SOC2). If the big companies fail to check that, then yes, they can be made liable.
Forcing function would be cyberinsurance policies that typically want to see audit results if you have multi-million dollar policy limits.
It will just be FTC knocking on your door…
It’s very basic. There’s no best practices clauses it’s all “reasonable” clauses. Also no requirement for an external audit.