I think, most of celebrities have enough money, luxury life, popularity, and so on. Most of ppl need those thing in life, then ppl use marrage is to fulfil that into some extend. Which means, when someone do not have money he prefer to marry a rich one. When someone wants to be famous, he or she may search for a famous one.
Since celebrities have money, popularity & others, they do not except those by their life partner.
Other thing is, most of celbrities are very busy persons, if they marry another busy person who is another celebrity, they may not have any free time for marrage life. I think, celebrities may also think this way as well.