Honestly, as someone else said, US based company, US based standards. Plus, youtube is really designed to show content designed for youtube, not old media from other platforms.

Even the BBC uploads old 576 content to iplayer as 720.
Because if you’re going to do the de-interlace etc before uploading, you might as well put it in a format that everyone is expecting.

It’s weird though, I swear I had a memory of 576 content being an option at some point…Maybe it was just some content being 50Hz.

Because America and NTSC use instead of PAL would be my guess.