Marketing agencies can only guarantee the things that they can control. So if their mandate is marketing, to build assets, creative, messaging and distribution. These are the areas they know they can perform in. Once a prospective customer goes into a funnel, or hits a product store, there are then elements out of their control.
That’s not to say it can’t be done, but companies have to be realistic. Where are agencies apply their work, and what is the best way to measure that. Good marketing should help lift sales, but it also adds value in other ways.