Many brokers, including IBKR, set their interest rate to be a small increase over the federal interest rate.
The federal rate is currently 0.25%. They set this rate extremely low to encourage the exact behavior we are seeing here - investments in the US economy.
I have a 1.4x leveraged portfolio with similar reasoning.
Worth noting: the implied interest rates on other forms of leverage (futures especially) are probably lower still. And though using long-term in-the-money call options might be a bit more expensive, they have the nice quality of having limited downside. If the underlying tanks hard you won't be forced to sell, and you might even still have some extrinsic value.
