You still need to manage the underlying database and security which requires a lot of knowledge to do well.

correct, and managed solution will have cost too, quality issues: 3p services have bugs and are hacked too and that's something you can't control, and added complexity. To me it looks like if you have small traffic/storage requirements, then cloud may look good. But if you have lots of data and need compute then running beefy bare-metal server can be very cost beneficial.

I still think that you need to manage some security things even if you are using AWS.