What standard library uses curl for HTTP ? AFAIK, none of Go, Python or Node does.

Many of the commonly used Ruby HTTP gems use libcurl under the hood.

I'm not that familiar, but I believe php does

"PHP," he said with a shudder.

I wonder how many attempts to rewrite curl would need to use curl to understand how the protocol works?

This is quite acceptable, IMNSHO. If you're reinventing a wheel, of course you need to know why your predecessors have not used the Obvious Simple Shortcut That Intuitively Makes Sense. Was it not available back when this was written? Was it to keep compatibility with Windows XP? Or perhaps your idea really did not occur to the developers.

Most time - when reimplementing stuff - is not spent coding, or even debugging, but on research.