For instance date formats are a complete mess only because people value different informations. Even in a countey with a single official representation, people will write checks with shorthands and mixing of different norms.
It’s also interesting to look at China would try to simplify pretty hard, and still ended up with a tangled mess (https://en.wikipedia.org/wiki/Time_in_China)
Suffice it to say, not only are you 100% right, but there are many easier and better systems we could use; and a software-defined world makes that actually easier than ever to implement in real life.
But people don't like change, and the biggest obstacle historically has been religion — depending which culture/country, pick one or two who oppose any change whatsoever.
Governments just don't see much incentive in doing anything either, because it's a losing proposition — you'd spend a lot of "political capital" and probably earn a lot of resentment in return, except for a few nerds who'd love it.
I've thought long and hard about how to overcome all these historical roadblocks, but I honestly have no idea in this case. Calendars are... loaded topics for way too many people, and useless concerns for most everyone else.
It's like the dozenal society. They're right, about everything, but it just won't happen.