One big difference here is that while the tools might all depend on the SDK provided by the cloud, the tools themselves can also do a whole lot of good/wrong on their side. Terraform fixes a lot of that by delivering a 'standard' provider interface with normalised data formats, resource structures and encapsulation. That was pretty much a requirement for the tool to work anyway, otherwise it wouldn't have a unified way to check for configuration drift, plan changes, apply changes, do cleanups etc. You also wouldn't be able to pass data around easily (you'd end up shuffling strings around instead).

Some people prefer to do the CDK thing where you use a general programming language to synthesise the IaC stuff and then run it that way, but that doesn't really fix anything because a CDK is just built on top of the same SDK. As an added insult to injury, you now don't have a domain-specific language so save you from yourself (and your team) with all the anti-patterns you now have at your disposal ;-)