I'm as big of a Hashicorp fanboy as you will ever find for on prem implementations -- I've used Consul, Nomad, and Vault. But that's simply not true. Here is an example of a Terraform template straight from their getting started page.
There's nothing preventing you from using other providers as well. You just have to understand how to achieve the same results when you build your modules.
So you have to rewrite your modules for each platform so you're still tying yourself to one vendor and that still doesn't alleviate risks from migration or the large amount of regression testing -- just in case Amazon goes out of business or you can might be able to find an equivalent slightly cheaper?
If you are a small company, the difference isn't worth the risk and the expense. If you are a large company you can probable negotiate with Amazon.
That doesn't even take into account all of the dependencies that your developers have taken on AWS services....