> Better yet require YYYY-MM-DD

This is the equivalent of requiring all your text to be in Esperanto because dealing with separate languages is a pain.

"Normal" people never use YYYY-MM-DD format. The real world has actual complexity, tough, and the reason you see so many bugs and problems around localization is not that there aren't good APIs to deal with it, it's that it's often an after thought, doesn't always provide economic payoff, and any individual developer is usually focused on making sure it "looks good" I'm whatever locale they're familiar with.