They are on some level separable; the problem is that there is a cost to the separation, and it's unclear whether we want to encourage everyone paying that cost by default (what is in the standard is automatically the default in a sense).

You can technically compose std::fwrite from std::putchar, but that's not a great motivation for only providing the latter. In fact, it's quite reasonable not to provide the latter at all. In fact, the only way to write to a file descriptor in the WASI is using an fd_write which takes an array of pointers to contiguous buffers, so it can do even more work in one call than std::fwrite.

In short, just because it may be academically satisfying to compose from smaller utilities doesn't mean that it's good engineering. I'd rather only have WASI fd_write() instead of only having putchar(), and I'd rather have views::from_big_endian_bytes instead of views::from_big_endian.