Because most projects involve interfacing with other third-party libraries that will undoubtedly not know about this other third-party library that implements a nice string.

I think nearly every C programmer has gone through the phase of Oooh I'll write my own string library! Sure, that works. Except you have to call system libraries and all kinds of other external functions all of which naturally assume the conventional char arrays. So you spend a bunch of time converting back and forth until eventually realizing it's silly, just learn the convention and go with it.

There are a large number of those libraries. Every large C project eventually seems to grow its own string class.

... except for libc, which apparently which is hardly ever questioned.

>>> Surely someone could write a better string library in C that hides the complexity.

In short, it's not possible to write a nice string library in C because C simply doesn't support objects, and by extension doesn't support libraries.

Strings are a perfect example of an "object" in what is later known as object oriented programming. C doesn't have objects, it's the last mainstream language that's simply not object oriented at all, and that prevents from making things like a nice string library.

If you're curious, the closest thing you will see in the C world is something like GTK, a massive C library including string and much more (it's more known as a GUI toolkit but there are many lower level building blocks). It's an absolute nightmare to use because everything is some abuse of void pointers and structs.