it's a history thing: back then, everything was serverless^WCGI and you didn't need to do any http servery things at all: just read stdin and write to stdout. web servers were an afterthought and turns out if you needed performance, you had to do it in C anyway, so nothing came prepackaged. then the web 2.0 happened and suddenly hello world is being done over http instead of printing to stdout, so new entrants obviously designed their stdlib around this.