Plenty of service providers (including OpenAI) offer you the option to kindly ask them not to, and will even contractually agree not to use or sell your data if you want such an agreement.
Yes, they want to use everyone's data. But they also want everyone as a customer, and they can't have both at once. Offering people an opt-out is a popular middle-ground because the vast majority of people don't care about it, and those that do care are appeased
They will do it when they need the money and/or feel they have the leverage for precisely the same reason that 99% of people won’t care. It’s better to assume they’re just sitting on your data and waiting until they can get away with using it.
That's nice. How can a user verify whether they fully comply with those contracts?
They have every incentive not to, and no oversight to hold them accountable if they don't. Do you really want to trust your data is safe based on a pinky promise from a company?
You sue them and win damages? Courts tend to uphold contracts at face value.