Why do binary files contain text but text files don't contain binary?

[Costello, Roger L. via Unicode]

Hi Folks,

There are binary files and there are text files.

Binary files often contain portions that are text. For example, the start of 
Windows executable files is the text MZ.

To the best of my knowledge, text files never contain binary, i.e., bytes that 
cannot be interpreted as characters. (Of course, text files may contain a 
text-encoding of binary, such as base64-encoded text.)

Why the asymmetry?

/Roger