It's much harder to appropriately address why such an academy doesn't exist in one place rather than why it does exist somewhere else. It's analogous to trying to prove a negative. The real answer is simply that no such academy was ever founded, because there was never enough motivation/pressure/etc to instigate such a founding. That's probably not a satisfying answer.

In some cases where one does exist, for example in China and in Taiwan, there was definitely pressure for such an organisation to exist as a means of national unification of a place that was otherwise linguistically incredibly diverse. In this case it was a few very specific efforts to standardise the language in the 1910s, 30s and 50s. In other cases, the formation of such a standard (and a governing body of that standard) is a major step toward perceived legitimacy of a new government (Bangladesh) or minority group's governing body (Cherokee Nation) or a regional identity (Assamese). You'll also notice that the Spanish language academy is listed on that page plus "21 other academies" in the Spanish speaking world. If Spain has one, and you're Bolivia seeking to legitimise your own variety, then there's greater justification in creating your own academy.

It's largely trends in broadcasting that have taken the lead in what's considered the prestige dialect. This is a big reason RP, what was formerly considered "standard" British English, is also known as "BBC English". However aside from the fairly midwestern "General American" English that dominated American news broadcasts until somewhat recently, there simply wasn't ever significant enough motivation to standardise beyond the natural (and sometimes less natural, thanks Webster) trends of what 's in vogue and what isn't. Also remember that the US has no official language, nor is there a single official standard form of the language. You can't have an official regulating organisation of the national language if you don't have a national language.

edit: I just realised you asked this three months ago. There was other activity that made it show up on the comment feed which brought my attention to it. Sorry for answering so late. I hope it's still helpful.