Why are there many Chinese people abroad who cannot speak Chinese?

There are not many "Chinese who don't know Chinese".

Many children from Chinese families cannot read or write Chinese, but they can understand and speak some Chinese.

Many people who went overseas in their early years mostly spoke Fujian, Chaoshan or Cantonese. , cannot speak Mandarin (Mandarin), but can read Traditional Chinese

Locke can speak Cantonese Taishan dialect and can read some Chinese, but cannot speak Mandarin.