There is an unused cloud service burning money at one of my jobs. Tens of thousands kind of money. Make a case for me even bothering to tell someone about it.
You can’t.
If management discover that you knew about the wastage, and did nothing, then it can be said that you aren't acting in the best interest of the company. Possibly the opposite, meaning you'd be fired. So there's that.
If there is a culture of apathy and zero oversight and you are 100% sure no will ever care to show that you knew about it, then I pity your situation. Not bothering to tell someone about the wastage it is perpetuating that culture. Eventually company will fail or at best struggle.
I'm my experience it's nicer working at profitable companies where everyone is doing their best to help the company to profit. That doesn't mean your income is less important than company income. But you agree on the former, then do everything in your capability to help with the latter.
If your still not convinced please consider the environment. Cloud services consume not insignificant physical resources.
If it's something simple like an unused service, I'll just shut it off.