One, it is not targeted. The Fed can pump vast quantities of liquidity into the heart of the credit markets in a way private actors aren't set up to.
Two, it is not fast enough. Companies will go bust while waiting for politicians to authorize a cheque, the Treasury to cut it, investors to cash it and then to identify and decide upon an investment.
Three, it isn't reliable. Investors trust the Fed's discount window will be open in a crisis. That knowledge, itself, stops many runs before they start.
Four, it's riskier. Helicopter money cuts cheques. Once it's done, the money is out the door. The government is never getting it back. If they sent out too much, we'll have inflation. Monetary policy involves collateralized loans. The collateral limits downside. And unwinding a loan is easier than raising taxes to drag surplus cash out of the economy.