Because most things in healthy programs aren't APIs, but just plain data: records, sum types and composites thereof (arrays, stacks, queues, whatever).

Fred Brooks:

> Show me your flowcharts and conceal your tables, and I shall continue to be mystified. Show me your tables, and I won’t usually need your flowcharts; they’ll be obvious.

In that situation, having to sprinkle "public"/"pub" all over them is very noisy and unproductive. It's the worst with Rust's enums, which require you to declare the visibility of variants and enclosed types of variants separately, so you see a couple "pub"-s per line of an enum definition.

That may be an argument for separate keywords for plain data, and for "APIs", although I'm not sure how I feel about it. Underneath it's just coupling of data with functions. Some languages have separate keywords for those, some don't.