Because that slack is the product. Without it, an agency that closes three accounts in a week gets told to wait — which makes the whole proposition worthless, since the reason to use a bench instead of hiring is that it absorbs exactly that spike.
The alternative model is running at 100% allocation and treating every new account as a queue entry. It is more profitable per head and it is why most fulfillment vendors cannot publish a turnaround.
This is the honest trade in our pricing: you pay a premium over a fully-allocated vendor, and in exchange the answer to "can you take this on Monday" is yes. If your account volume is flat and predictable, that premium is not worth paying and you should hire.