For me it's because people already know and are already using Postman. We already author collections for services that other devs in the company reference when using our APIs.

Sure we could write the tests in python or whatever, but everyone is already using and likes Postman for the most part. And replacing the postman collections that people use for reference with a folder of scripts I don't see going over well.

I'd personally prefer something like https://hurl.dev where you have a readable file in source control, but that's not a battle I'd win at this point.