> Sure, but you can buy US treasury bills, and then your risk is "lose some money if the US government defaults", which is very low.
There are all kinds of other risks associated with buying US treasury bills besides the US government defaulting, which is why you get paid - but you're right, the risk is low so you get paid a low amount. You have opportunity costs during the time that your money is locked up in treasuries. You also incur some inflation risk. If we are specifically talking about T-bills then we're talking about treasuries with maturities of less than one year, meaning that particular risk is low. There is interest rate risk. If you have an emergency and need to convert back into cash before the maturity date hits, you have to sell them on the market, where you may lose money if interest rates have increased. If you're investing in the treasuries via an ETF or via a broker, you are incurring additional counterparty risk.
> If you look at the current treasury yields, they are very close to 3%.
The 10-year bond is 2.91%, the 20-year bond is 3.05%. Investing in a 10 or 20 year bond is obviously different than having a checking account which can be emptied at any time without penalty and without having to go to the market to find a buyer, so there's a large maturity mismatch that's being incurred by your counterparty, RobinHood (assuming they are in fact investing in long-dated treasuries).
> If they go bankrupt because of an unlikely combination of events - lots of deposits coupled with a very sharp and unexpected decline in treasury yields - SIPC will pick up the pieces and make sure you get your cash and securities up to $500k.
The part where they pick up the pieces could take weeks or months; if you need the cash before then, you're in trouble. If you can afford to wait, you're right, no big deal. I don't expect RobinHood to go bankrupt tomorrow, but if they were wiped out as part of a wider financial crisis, it's possible that under those conditions you'll need access to your cash quicker than you think.