Most of that money was their customers' money (e.g., money for payroll). You can't just go invest your customers' funds. Well, I guess you could, but I expect there are tons of regulations. And you'd potentially end up in the same exact situation as SVB: not having enough liquidity to pay out your customers if they ask for their money. Or, your investments are underwater and you don't actually have enough assets to cover the funds your customers have entrusted you with. (Certainly, banks do this, but you're also usually not paying the bank to hold onto your money for you. Whether banks should be allowed to invest your funds at their discretion is another conversation)

Should Rippling have spread those funds out over multiple banks? Maybe. But more moving parts means more room for error, more complicated accounting, and more operational overhead. There is (was?) a compelling argument that you're more likely to goof up juggling $300M than a bank like SVB is.