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.

Because who the hell wants to say it's 11 in the morning while it's dark out?

Milliseconds since the epoch is the only true time

I'm now imagining that playing out.

"France, we're thinking about adopting British time as the global standard. Do you have any thoughts or input on the matter?"

Because the vast majority of people aren't terminally online and/or affected by timezones.

Most people don't have to deal with booking a meeting a few timezones away or anything else where it would be an advantage on a regular basis.

It's convenient if the date, and possibly weekday, changes at night.

Why should the UK get to be the only place with an accurate local time? I don't want to live on UK time.

Same reason some people use miles instead of kilometers, or that most people use Windows even if they hate it.

Inertia is a powerful force.

Swatch Internet Time tried doing something like that