I feel that most people instinctively assume that some institution, e.g. the government or the dictionary publishers are the authority on what constitutes "correct language". It's important to emphasize that language (included spelling) is something that develops organically and that the role of these institutions is just to capture the status quo. At least that should be the case in free societies. Language is power…
This has historically been the philosophy of English linguists, but for many languages (Spanish, French, German…) there is a central institution that does indeed decide what is officially correct. Their decisions are taken seriously and are intentionally propagated anywhere where language is used in a somewhat official context (not just in public institutions). True they adapt the standard over time following common…
It's not unique to English linguists, it's a tenet of modern linguistics in general. A language is defined by the way people actually speak. If that's influenced by a central organization, fine, but that does not contradict descriptivism at all. Someone studying a language should always study the way the language is spoken by real people, using prescriptivist sources as supplementary sources of information where needed.