1. Lots of people speak Mandarin at home. No one speaks MSA at home except us poor language students.
2. The set of languages binned under 'Chinese' is much more diverse than the Arabic dialect continuum.
3. The writing system is shared between several languages. You can write Cantonese (kind of) in hanzi and write notes that Mandarin speakers can read even though the languages are totally mutually unintelligble.
4. There is no dominant dialect of Arabic, like there is a dominant variety of Chinese. Instead every dialect shares the same formal register (MSA) which is taught in school like a foreign language.