Well for one you're able to easily use macros to customize features you want and don't want. Plus "forcing me to figure out the specific XYZ_IMPLEMENTATION" is a big of a weird compliant. You could say the same thing about linking a library. Besides, it's not hard to figure out and is usually one of the first lines of the file.

The single-header format also gives you MORE ways to compile the library and control which features to use or not use.

I am not say it is hard. But I find having a .c/.h pair more useful and easier to work with.