The deeper cost is not the fee. It is that the account, its history, and its data are never yours. It works a lot like the managed website builders that host your site on their platform: it looks like yours until you try to leave, and then you learn you cannot take the pages, the traffic, or the search authority that built up on it. A rented ad account behaves the same way.
And an ad account is worth more the longer it runs. Google's and Meta's systems spend months learning who your real customer is, which searches and audiences actually book, and where the money is wasted, and they get sharper at reaching those people over time. That compounding is the real asset. If the account belongs to someone else, the compounding belongs to someone else too. Walk away and it stays behind. That is a switching cost built in by design, not by accident.
An independent agency does the opposite. The account is yours from day one, the learning compounds in your name, and if you ever change partners you keep the engine, the history, and the head start.