[smallnamespace]

> Their written form are all the same.

Regional languages in China have separate writing system when you need to write what is actually spoken.

https://en.wikipedia.org/wiki/Written_Cantonese#Vocabulary

https://en.wikipedia.org/wiki/Fuzhou_dialect#Writing_system

Easiest way to see this difference is watch the news in Cantonese -- it is mostly 书面语 (standard) mandarin but pronounced as Cantonese, whereas day-to-day spoken Cantonese is completely different in structure and vocabulary. What do you think gets written in HK movie scripts? Definitely not standard written mandarin.

[FabHK]

Yes. It's a bit as if French, Spaniards, and Portuguese would all correspond in Latin, and then say, see, it's one and the same language :-)