C unions are sum types if you just "draw the rest of the fucking owl". C has sum types the way x86 assembly has function calls. You see, there are "call" and "ret" instructions. Just figure out calling convention and the stack yourself, it's easy. And yes I can stick the tag next to the union myself in C and write a bunch of functions that hopefully deal with it correctly. I can even write some macros that look like pattern matching and maybe convince myself that it's a language feature, but it just isn't.

And it's very evident why the difference matters. People gravitate to what's readily available and ergonomic. That's why every C API under the sun (most definitely including libc itself) does janky stuff like returning -1 or "*" on error instead of building their own bespoke Result-style abstraction. A programming language is not just the things that are in some way possible within it, but more importantly the things that are easy in it.

not in a way that forces the user to actually handle them properly. C is a weakly typed language and as such is pretty badly suited to systems programming.