A big practical difficulty is the pressure for languages to continue to evolve in different directions when speaker communities are even slightly separated (hence the difference in present-day varieties of English, which is probably growing larger over time rather than smaller, based on (almost, approximately) a single common ancestor just a couple of centuries ago in the time of the British Empire and colonial expansion).

People can decide to try to prevent dialects from forming and diverging

https://en.wikipedia.org/wiki/Language_planning

but they have to actively work against it if they want to prevent it, and a lot of the common means of doing so historically have been pretty harsh (e.g. corporal punishment in schools for using disfavored varieties). I guess this is partly because the consensus that we should centralize and unify and standardize language, for all of its logistical and economic and cultural benefits, is mostly a central government/imperial metropole impulse rather than something that the general public is super-attracted to.

Standardizing language and letting it diverge both have cultural benefits! Just different ones. The standardization benefit is that more people can access a much larger body of culture without linguistic barriers and without needing translation or explanations, and cultural conversations can grow larger, with more direct participants in a given conversation or genre. The diversity benefit is that the language differences themselves can acquire, or mediate, cultural phenomena and cultural creativity.

English doesn't have a significant tradition of language planning the way that some of the continental European languages do, and this is posing a (minor) challenge to English's role as a global language, as there are sometimes genuine difficulties of intelligibility between English varieties, and again I think those differences are getting wider on average rather than narrower. Then again, if we did have such a tradition, it doesn't mean everyone would care to follow it... it's not as though everyone in Latin America cares about what the RAE says (or that they refrain from having meaningfully different vocabulary and grammar between Latin American regions!), or even that everyone in Austria or Switzerland necessary cares what Duden says.