> After all, why would they have to convince you to invest money if they could just do it themselves?
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.
This works sometimes, but banks don't want to be overexposed (even if something is very low risk).
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.
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.