Probably because it's easier to buy something that costs $10/month instead of "maybe it will be $0, maybe it will be $300,000, we'll see at the end of the month!"
When you're using your own money to pay for cloud resources, that unbounded worst case is pretty scary.
But you have to admit that if you're just some scientist who wants to GPU-accelerate their Python notebook, "click here to pay $9.99" is a lot better UX than "just log into the Google Cloud Console and change 83 settings!"
When you're using your own money to pay for cloud resources, that unbounded worst case is pretty scary.