Because the defaults aren't just for convenience, the API designer is also making the parameters they think should be used the most have the least resistance. Good example is runtime parameters like in the JVM. You shouldn't start with having to tune your JVM, you probably want a middle of the road place to start with even if you know you're going to tune it.

Because LLMs degrade with context length. And even if they didn't, having to constantly ingest the same stuff is wasteful for execution and cost. Why stuff the context when you don't have to?