It's the historical interface which websites now rely on, changing it would be like writing a libc with strcmp operating on Pascal strings.
In any case, a Javascript String is not actually designed to be UTF-16, it is essentially just an `uint16_t[]`. Even textual strings just store UTF-16 code units, not full UTF-16 data. Relevant snippets from the standard:
The String type is the set of all finite ordered sequences of zero or more 16-bit unsigned integer values
("elements").
When a String contains actual textual data, each element is considered to be a single UTF-16 code unit. [...] All operations on
Strings (except as otherwise stated) treat them as sequences of undifferentiated 16-bit unsigned integers;
they do not ensure the resulting String is in normalised form, nor do they ensure language-sensitive results.
> Although the standard does state that Strings with textual data are supposed to be UTF-16.
No, it doesn't. It states that they're UTF-16 code units, a term defined in Unicode (see D77; essentially an unsigned 16-bit integer), which is not the same as UTF-16. A sequence of 16-bit code units can therefore include lone surrogates, which something encoded in UTF-16 could not.
