Capital. Taking a 5% cut of billions of dollars is going to be worth a lot more than 20% of whatever tiny amount of capital you are able to muster yourself.
There are ways of getting capital that don't involve the public's money.
There's the old saying "If you own the bank $1m, that's your problem, but if you owe the bank $100M, that's the bank's problem".
This kind of stuff happens in other industries (like real estate) all the time. Even with bank financing, you'll need another source of funds (typically LPs) to meet loan requirements.