1. Your spend is small and your time is cheap
Below a certain level the fee buys less than the learning is worth to you. Run it yourself, badly at first, and get good.
2. Your account is genuinely well run already
Some are. If tracking is right, brand is split, negatives are maintained and it clears break-even, an agency is buying you time, not performance.
3. The constraint is not the ads
If margin cannot carry a click, or fulfilment cannot take volume, paid media makes the problem arrive faster.
We turn down more stores than we take on, and these are the three reasons. Finding out on the first call is cheaper for both of us than finding out in month three.