It was quite weird before we wrote concurrent software. Once you're writing concurrent software it's very weird because as originally documented this single error location is shared by all threads, so it necessarily gets concurrently modified while being read, a Data Race.

Your 2022 C library typically defines it as Thread Local instead, so when you call some_c_function() and then check errno, you aren't confused by the fact meanwhile another thread tried to open("nonexistent.file") and so their thread set errno to report that the file doesn't exist.

Given that none of this fuss is how the actual system calls work, it's understandable that people might want to sidestep it.