the answer is because in a cash account you do not have the settled funds to purchase the stock. you are using the brokerage to pay for it. so doing it 3-4x over and over in a day you are risking/tying up more of the brokerages money prior to your cash settling. This is why they have margin accounts, not everyone is approved for margin trading for the above reasons, the brokerage will ask you income, asset, liquid asset information and make a determination of your eligibility. If your brokerage approves you for the margin account, you can do what you are looking to do.