I never understood why AWS doesn't provide something like LocalStack out of the box. Any team building serious software on AWS needs to mock AWS services in their CI/CD pipelines. What exactly are they expecting developers to do? They would probably argue something like "spin up real infrastructure so you are as close to production as possible" because this way they could make even more money while also avoiding the implementation / maintenance cost of the mocks.

My general principle has been “don’t build what you can’t test locally”. It seems so obvious but PaaS does challenge it.

It also has the benefit of steering clear of exotic proprietary features that are hard to migrate between providers.

Local stack formed a big part of making that principle realistic.

(EDIT - but I can see how that's counter to AWS' interests! It's desirable that they provide it, but not surprising that they don't.)