Because retainers pay agencies for being busy, not for being right. A retainer gets invoiced whether the account grew or not, which is why so much agency output is decks and meetings instead of winning ads.
With the performance model, the only way we earn well is if your bottom line grows. Street interviews and UGC are the exception: those are sold as fixed-price content packages, agreed up front, because there we deliver a defined creative product rather than managed outcomes.
