It’s because a lot of the way humans go about their life is based on traditions. Getting everybody to switch from a system that already works pretty well is just a hassle.
Examples:
English spelling is faaar from phonetic and children take longer to learn how to spell than in Spanish for example. (though, cough, enough, plough instead of something like thouğ, koff, enaf and the US plow)
Metric system adopted globally would streamline a lot of global industries that have no cater to each system.
Driving right side everywhere. Sweden switched but asking India to switch makes way less sense.
Date formats. Arguably the best if everyone uses ISO 8601 but nobody does.
It’s because a lot of the way humans go about their life is based on traditions. Getting everybody to switch from a system that already works pretty well is just a hassle.
Examples:
I do use ISO 8601