Why Chinese say "mai dong xi", but do not say "mai nan bei"?

In Chinese, if you buy something, you say "mai dong xi (买东西)". The word "dong xi (东西)” means something. 东 is the word for east [1], 西 is the word for west. So many people ask why not to say "mai nan bei (买南北)".? 南 is the word for south, 北 is the word for north. I don't know the reason. Who can tell me why?

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[1] Any of the following would work:

"东 is the word for east."

"东 is the word meaning east."

"东 means east."

You could also use punctuation and your own format:

"东 is the word 'east'."

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I find 东西 to be a very curious word.  I am not curious about why 东西, rather than 南北, was chosen to mean 'thing'.  I am curious about why 东西 means 'thing'.  东西 would seem better suited to mean 'latitude', or 'vast', or 'expanse', or 'everything'.  It seems it would be a word meaning something very big, not a word meaning some small 'thing'.