Why use cloud services from outside companies?

The government uses private companies for many reasons. It all helps with plausible deniability and makes any investigations more difficult.

When MK Ultra started looking promising they moved research from the CIA into private hospitals in Canada. Then FOIA no longer worked, the hospitals could claim that any information was propietary information and the funding route was obscured. It also created an "international" problem in that case as well.

It is like when you get your broadband internet service from someone who is not your telephone provider. They can both point the finger at each other if something goes wrong.

It is also a good way to pay back your friends. For instance, a newspaper like the Washington Post, say, might be better disposed to show the CIA in a good light if its owner is receiving $billions for a cloud service to the CIA.