Because the text they've been taught is the same , but what they say is not. The spoken languages have some common words but are actually distinct languages. Otherwise there are plenty of dialects which I'll ignore . I believe these range from literally different languages to basically speaking with an accent.

When you consider the difference between traditional and simplified written Chinese , traditional being used in Taiwan and Hong Kong , Vs simplified in mainland China , simplified is a "shorthand" or "easier" version of the same symbol, which once you know the "formula" for how this is done, is relatively straightforward to do in your head.