However, the languages evolved much faster historically than they do today. On one hand, the states are now effectively enforcing standard forms of language on populations via universal primary education, and then mass culture and media further reinforce that, often aided by social conventions (where the enforced standard often becomes socially proper "educated speech" that people strive to emulate to present themselves better and/or to not be discriminated against). And at the same time, modern borders significantly reduce migration rates, making it harder for language innovations to spread.

I'm not saying that it doesn't happen, of course. But a phonemic spelling created today is still likely to have a much longer useful lifetime than one created 300 years ago, say.