A record number of firms plan to cut advertising spend on X next year because of concerns that extreme content on the platform could damage their brands, dealing another blow to the financial fortunes of Elon Musk’s social media company.
Money has been spent, budgets committed, etc. Think of it like steering a large ship. You may turn the wheel immediately, but the ship will take time to adjust it's course.
And if they already paid for a certain amount of ads? What if there is a contract and they are obligated to have their ads on the platform for a certain amount of time? There are any number of reasons why they can't just pull the plug , no matter how much they may or may not want to.
Often there are contracts. Sometimes for a very long time, often multi-year. There are sometimes escape clauses (like a morality clause for a spokesperson), but these aren't easy to invoke.
I suspect many of them are up for annual review/renewal, when they can be terminated without penalty. It might also just be an attempt to get better terms.