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.