There's a lot of complexity in managing and even building the thing from the start -- and then you have to support it. If you're working in a large org then there's a chance that you can just DIY it, however for small-medium businesses this isn't practical and it's a waste of time (their most precious resource).
I like to think of it as a managed database. Sure, you can freely download postgres in a container and you're up and going, however there's a lot more costs to it than just that. Having a fully-managed database saves you time and other non-tangibles that it can be worth the cost. Just depends on your circumstances.