>So why do you or your company who uses any of the big 3 use them if you had to cut cost at some point?

Because it takes X hours of effort to cut costs and using another provider would have required Y hours of effort where Y >> X. Y may be greater due to reliability issues, missing features that you need to build yourself, training costs of new employees, etc.

edit: Also X is paid once you're succeeding, Y has to be paid before you're succeeding which makes Y even more expensive in opportunity cost.