Let’s take the Rhine Which goes from Switzerland to Netherlands. When and how did people start agreeing on giving-it the same name ? What about the alps going from southern France to Austria ? When did people agreed that it was the same mountain range ? And obviously what about the Gulf of Mexico ?
For the most part, whomever the mapmakers happened to ask got to decide. Once it’s on an official map, it’s just gonna drown out any other opinions.
Several places in Norway have names that are spelled “wrong” according to locals, but changing it requires herculean efforts of bureaucracy.