They don't need to. Robinhood is just a front end for Apex Clearing. Other apps like Tradier are front ends for Apex too. Apex gives up to 4x leverege before a margin call, so I'm just at 3.5x or something like that. They charge a bit more than the fed rate but less than 5%. Robinhood pays this on your behalf, without taking any liability for it. They then charge most people 5%+ in the convoluted flat rate gold tiers structure. They pocket the difference. Apex is the one taking on liability, but honestly 4x leverage is the norm for a good broker. It's not unusual. After 4x they liquidate your account to prevent it from going further. They're pretty safe.

All I did was find a way around Robinhoods trading block by buying then exercising close to the money options. Now Robinhood has to pay interest while I just pay $125 a month. They're losing money there but they aren't at risk of anything. On average they make money because most gold users don't go negative on buying power.