China had an imperial bureaucracy for over 2000 years, which sent officials from one end of the country to the next. In fact, a predecessor of Standard Chinese (a.k.a. "Mandarin") was called "the language of officials" (官話).

This is especially apparent when speaking to less educated people with less exposure to the standardised, official Chinese language, which is what people do actually write down when intending for a broader audience, of course. Diglossia is real.

Others have already corrected your other misunderstandings, but this is also false. Spanish has at least as much variance in pronunciation as English and has an orthography that is extremely regular. Brazilian Portuguese and Portugal Portuguese likewise have the same, highly regular orthography and are barely mutually intelligible.

To the best of my knowledge you actually have the causality mostly reversed: English's orthography is useless largely because the pronunciation changed but the spelling didn't, and English has a variety of pronunciations because the pronunciations changed differently in different regions. English has a messier orthography than other languages because of our complicated history of borrowing words, but the evidence shows that even people who start with a highly consistent orthography don't use it to keep their pronunciation static and shared.