Ask HN: Why don't monitors accept USB-C along with DisplayPort, and HDMI?

This one does. https://www.apple.com/us_smb_484614/shop/product/HKN62LL/A/l...

Your question is invalid. There are monitors with type C ports.

Usb-C is not mainstream outside the macbook pro market. Everybody else is quite happy with HDMI or even VGA still, and moreover, there's so much old hardware that's perfectly usable that uses those ports.