I've encountered the same phenomenon, and I too cannot explain why it happens. Some of the highest-value types are the small special-purpose types like the article's "CreateSubscriptionRequest". They make it much easier to test and maintain these kinds of code paths, like API handlers and DAO/ORM methods.

One of the things that Typescript makes easy is that you can declare a type just to describe some values you're working with, independent of where they come from. So there's no need to e.g. implement a new interface when passing in arguments; if the input conforms to the type, then it's accepted by the compiler. I suspect part of the reason for not wanting to introduce a new type in other languages like Java is the extra friction of having to wrap values in a new class that implements the interface. But even in Typescript codebases I see reluctance to declare new types. They're completely free from the caller's perspective, and they help tremendously with preventing bugs and making refactoring easier. Why are so many engineers afraid to use them? Instead the codebase is littered with functions that take six positional arguments of type string and number. It's a recipe for bugs.

structuring what lives where became easier, naming things became systematic and consistent, and writing unit tests became simple.