Because (on average) privilege kids eventually become soft, entitled, no-good jerks quite soon as generations pass. And even their education can (and often is) paid for. So much for merit. Many have had the "privilege" to witness the "intellectual capacity" of kids of the privileged classes in action...

To quote an article I've read:

  The Chinese have a saying: “Wealth does not pass three 
  generations.” But it’s not unique to China; the same 
  sentiment is expressed across multiple cultures, from 
  Japan (“rice paddies to rice paddies in three 
  generations”), to Scotland (“the father buys, the son 
  builds, the grandchild begs”) to even here in the U.S. 
  (“shirtsleeves to shirtsleeves in three generations”).