You may never have dug that deep into their respective documentation, and you may never have even heard of libcurl, but their superficial overlap in ability to download a single file from an HTTP server does not make them "trivially replaceable" with each other (in either direction).

Regardless, even if hypothetical product X could do everything cURL and libcurl do for most people, that wouldn't make cURL/libcurl much less load-bearing, any more than the existence of FreeBSD or Illumos make Linux less load-bearing.