"This is what I want from infrastructure as code and I have yet to see it."
If you sit down with the terraform specifications for an AWS instance, a GCP instance, and an Azure instance, and start trying to write that harmonization, you will rapidly discover why for yourself. Even just trying to specify a network setup and putting an instance on the public internet is impossible to harmonize, without making something so lowest-common-denominator it is almost useless, let alone anything complicated.
If you sit down with the terraform specifications for an AWS instance, a GCP instance, and an Azure instance, and start trying to write that harmonization, you will rapidly discover why for yourself. Even just trying to specify a network setup and putting an instance on the public internet is impossible to harmonize, without making something so lowest-common-denominator it is almost useless, let alone anything complicated.