Because the payment card industry and the banks that make up the PCI, aren't going to eat losses due to fraud, chargebacks, and refunds. So you must hold a reserve to cover them, and it's either your money, or the merchants' money.
If you're holding the reserve from your own pockets, you're very trusting, to the point of being naive.
Additionally, the longer you hold the money, before sweeping it to the merchant, the more interest you earn on it.