Ask HN: Why are managed database services so expensive?

I noticed that on virtually any platform that provides managed databases (Digital Ocean, GCP, AWS, etc etc) has non-linear price increases. I use a managed db service personally and it's easily 90% of my infrastructure bill. Why is that? What's going on under the hood? Is it really that costly for a service provider to provide a reliable and safe db?

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Edit; I was referencing specifically postgresql, mongodb, and mysql