As a case in point, I worked in standing up a critical system in a large enterprise a few years ago. We spent about $12M on compute, storage, networking, etc. At operational state, it was about 40% cheaper than AWS. The problem is, it all sat there for 6-18 months filling up before we fully hit that state.

With a cloud provider, you pay a high unit cost but if you engineer intelligently your costs should move with utilization. Except for government, most entities generally want to see opex move with revenue and prefer to minimize capex where possible.