I’m going to limit this to lexicon, and not get into other areas of language change.
Think about it. You just spoke of scientific terms being planned out meticulously and promoted by universally acknowledged authorities. Scientific terms are part of language. That includes smaller languages’ authorities, which come up with canonical translations of other languages’ scientific terms.
Why doesn’t that happen in general with vocabulary? It’s not like there’s a shortage of authorities wanting to run languages.
First, the only function of scientific language that scientists consciously acknowledge is referential, describing the world. (There’s other stuff they don’t acknowledge, such as group identity.)
But language in general does a lot of stuff, that a central authority is simply not going to be able to predict. And language is a spontaneously changeable instrument, which authorities simply can’t intervene in in practice. It’s the major vehicle of interpersonal relationships, and of individual expression.
Even if it did, language is transmitted person to person, and evolves through a network of influencers and followers—being a social phenomenon. Any central authority is going to be only one of the influencers. And not everyone is going to choose to follow everything a influencer does.
Which makes it ultimately a democratic-ish outcome.