Usually because the company already pays an AWS bill. No one will care if you add another Lambda function, but to use an alternative you’ll have to get past some gatekeepers.
Does the project have a testing framework in place? How is the project built and distributed? Can testing be integrated into its CI/CD pipeline? Is there a CI/CD pipeline? Adding frameworks and infrastructure to a project is one of those things that is a lot more work than it appears to be. They should do it, for sure, but I wouldn’t dismiss it as trivial on a decades-old system that was never built for it.