Indirect rates are negotiated. What are the incentives for the government negotiators to get the lowest possible rate? I honestly don't know; I'd like to understand more about the underlying drivers here.
And it's also why it's so insulting to just dismiss this whole process and claim rampant fraud with zero evidence. It's basically accusing thousands of people of being lazy and felonious and maliciously negligent. It's telling thousands of researchers who struggle for funding that they're apparently just morons for not getting in on the "infinite cash" we purportedly hand out like candy. It's saying universities don't need buildings or janitors and shouldn't take bulk discounts on their raw materials. Etc.
This isn't even getting into the absurd misunderstanding of economics of treating a government grant like a hamburger purchase. The US gives money to a US university which purchases US materials and pays US staff that are all taxed to give money to the US government. And their findings develop knowledge and cultivate skilled labor in the US that produce higher value exports for the US which are taxed to the US government. They aren't buying a hamburger, they're taking on what has proven to be one of the best investments in world history.