Tangential, I have always wondered why compact or pocket dictionaries (the single language ones) contain the simplest words. If space if of the essence, why not

Tangential, I have always wondered why compact or pocket dictionaries (the single language ones) contain the simplest words. If space if of the essence, why not skip the words that everybody knows (like "table" or "shoe") and use the saved space for difficult words? After all, those are the ones you are likely to look up.