You meet the senior person in the pitch. Then the work goes to someone three years into their career, following a template, reporting on metrics that flatter the retainer. Nobody is lying — the model just does not put senior attention on your account after month two.
The alternative most companies try is hiring. A good marketing manager is affordable and cannot set strategy. A good marketing director sets strategy and costs more than the problem is worth at your stage.
So you choose the level instead of the headcount. Buy execution when you know what you need. Buy leadership when you do not. Change level as the business changes.