But it doesn't save much engineering time. You now need to manage those services, and the high mark-up means you need to invest effort in scaling things up/down.
Yes, it took me about a week to learn to set-up a Postgresql high availability cluster. But now it saves me $4,000 per MONTH for each of our 10 databases.
And if you are using EC2 instances, AWS saves very little effort compared to bare metal.