If it were so easy to make samples and 
  they were so disposable that anyone 
  could come up with them, then there 
  would be no need to sample older work 
  and no one would do it.

You don't use samples because it's difficult to create new ones. You use samples in order to evoke shared cultural heritage.

Imagine I write "I shot the sheriff" or "That is not dead which can eternal lie" or "x considered harmful" or "I've got 99 problems" or "We few, we happy few" or "Beam me up, Scotty" or "Light of my life, fire of my loins" or "Of the people, by the people, for the people" or "Less space than a nomad".

I'm not just stealing a sequences of words I think sounds good. I'm referencing other works - partly to evoke their meaning, partly so when you recognize the reference it communicates that I am like you, that we have shared background, that we've read the same books, heard the same songs, visited the same websites.

Is taking a few notes from a song really so different to taking a few words from a book, speech or essay?