This is only because almost all native Cantonese speakers are trained to read and write in Standard Chinese from the very beginning.

Standard Chinese can be spoken in Cantonese, but while understandable, it is never done, in contrast to Mandarin.

In recent years written online conversations (like blogs, Facebook posts, instant messages) by Cantonese speakers are increasingly written in Cantonese instead, and this is slowly spreading to more "formal" media such as books.

Source: I'm a native Cantonese speaker.