Companies choose providers and tend to stick with them, but people don't always stick with companies. If I know TF there is a decent chance my skills will be applicable when I change companies.

Also some big corps run their own internal datacenters and have cloud-like interfaces with them. You can write TF providers for that (its not going to be as nice as the public cloud ones, but still nice). Then you can utilize Terraforms multi-provider functionality to have 1 project manage deployments on multiple clouds that include on-prem.

Also terraforms multi-provider functionality is also useful for non aws/azure/gcp such as Cloudflare. As far as I know CDK does not support that.