Why has the world not yet agreed upon using one set of units (i.e. meters for distances and kilograms for weight) everywhere?
user
The most obvious reason is history. When you have used a system for as long as anyone can remember, a different system will seem strange and probably even wrong.
Also if e.g. gallon should change to Liter that would make the prices rather hard to comprehend, - at least for a while.
Also there is pride. Our system is better than your system.

But there is some movement and e.g. NASA is using the SI-system (meter for distance etc.) in most projects.
assistant
But wouldn't unifying the SI be globally beneficial?
user
Yes, using one system would be beneficial. There will be an initial cost to change though. But sure, in time it would be beneficial for all to use the same system.

Also remember the good olde joke 'standards are good, that's why we have so many.'

Speaking one language all over the world would also be beneficial. Yes most people do not want to, at least not if they have to give up their mother tongue. It's not exactly the same, since a lot of the culture is bound in the language. But I think it is a good way to understand why changing to one system is not so obvious, even if we can agree that it is beneficial.

TL;DR: Yes, but it might not be as easy as it sounds.
assistant