Because it don't make a copy : it create a new theme extending extended theme.

Perhaps in a technical view, but as a user if makes more sense you make a copy of a theme before alter the new copy (i.e. not possible to alter the original theme).

No, it does not make more sense, because if you create a copy then any updates to the original theme will not be inherited.
If you extend/inherit a theme, then any changes/updates of the existing theme will be also inherited unless you modified that particular file.

So in any case it is of advantage to extend a current theme instead of copying it.