Likely done by an eager intern back then as a binary drop and never properly integrated into whatever build system Windows uses.
Nevertheless, as a consumer, I appreciate having curl in-box (no matter how out-of-date). Came in handy when I wanted to firewall the heck out of Windows 11 and strip out every unwanted component (including the only browser, Edge). Of course, later found out that curl.exe complains about ssl certificates if you firewall / block whatever Windows service does certificate validation, but luckily PowerShell has a built-in wget command as well for downloading which doesn't seem to care as much :)