I work for a company on the list (Aptible), and we do not use Terraform for our customer-facing infrastructure. We experimented with that approach a bit, but I didn't love it. The primary reason I didn't love it is because Terraform itself is prone to flakiness and failures that requires a real human to clean up. This doesn't matter if you're doing it on a small scale (i.e. if you're an SRE and it's just your infra), but doing it at any large scale for customers it just didn't make sense. I do think there's likely a path forward with more flexible tools like Pulumi and CDK, but those are going to have limits in terms of what they're capable of too.
I know less than I'd like about what some of the others on that list are doing, but for a majority of them, they're heavily k8s/Nomad based (which, we also aren't that either, but it's also something we've been poking around at), which lessens the dependency on IaC after you have the initial cluster up and running. Fly.io also has a problem which I consider potentially even more interesting, which is that they run their own severs, so most IaC doesn't even make sense.
Co-founder of Porter (https://porter.run) here - we do not use Terraform under the hood. We moved away from an IaC based system earlier this year to better manage our users' infrastructure distributed across multiple cloud accounts. A decision that definitely turned out to be conveniently prescient :)
With this new system, we are also able to immediately reconcile drifts that occur in our user's infrastructure, which an IaC based system did not allow us to do.
As other replies have mentioned, Terraform is challenging to create a smooth experience. We have spent many cycles working on smooth orchestration so that it's fast and intuitive. Our primary motivation was to avoid hiding infrastructure behind proprietary technology.
We have support for other IaC tools on our roadmap so that each team can use what is comfortable.
I know less than I'd like about what some of the others on that list are doing, but for a majority of them, they're heavily k8s/Nomad based (which, we also aren't that either, but it's also something we've been poking around at), which lessens the dependency on IaC after you have the initial cluster up and running. Fly.io also has a problem which I consider potentially even more interesting, which is that they run their own severs, so most IaC doesn't even make sense.