To make a python metaphor, if cpython had a testing suite but pytest didn't exist then people wouldn't be able to test their own python code. That's kind of the situation with Terraform right now- you can't test your code using just the Terraform tools, you have to rely on Terratest which was written by Gruntwork. Hashicorp has spent years relying on the open source community to fill those gaps, which Gruntwork has done very nicely.

Hashicorp even recommends Terratest for testing: https://www.hashicorp.com/blog/testing-hashicorp-terraform