"I just can't fathom why large companies agree to take that kind of risk"
They don't see it as increased risk. The IT department is typically not hugely respected - to the extent that an IT director practically never gets a seat on the board, so replacing the IT department with a "service" provided by a "major player" makes apparent sense (particularly as you can "downsize" the staff). There's also the very common misapprehension that by outsourcing the activity you can palm off the risk on the provider. I have on numerous occasions tried to get the message across that you can't, but it's always been a hard slog with limited assurance of success.
In my experience, the real benefit of cloud (dynamic performance and capacity flexibility) is hardly ever the actual reason for adoption.
Biting the hand that feeds IT
