The Question is nothing new to wonder about. It has crossed almost every sane mind, but the answer to this can differ depending on various reasons. Celebrity relationships also go through ups and downs like any other ordinary relationship. One thing that I believe in any A-grade celebrity will not give up their relationship just for media highlight. There are some theories made upon the same.
Busy schedule
Celebrities are often packed up in the tight work schedule that protects them from being with their partners. This reason can create a communication gap and distance between the couple. This can also cause a number of problems like one of the partner may get tired of continues trying to reach out to other and failing, lack of intimacy, lack of emotion support etc.
Higher Temptations
We cannot deny the fact that a number of people are hitting on you a celebrity all the time, be it another celebrity or an ordinary person. The celebrity is always going to know that someone or other is wanting them all the time, so even a tiny problem in their relationship drifts them away to other people where they do not have to put in any effort.
Age Difference
Celebrities generally do not care enough of the age of their partner before hitching. Age is not just a number like they say, age actually determines the thinking process of a person. This is why many celebrity couples having age difference fall apart due to the difference in their opinions.
Narcissism
I would not consider it as a fault of the celebrity. If you they have thousands of people waiting just to look at them once, screaming their name everywhere they go, will definitely make them feels like the ruler. Then they might start thinking of themselves s the supe4rios one in the relationship and fall In the trap.
Culture
They generally fall for each other on the basis of their outer image in public and industry, but the cultural difference is something that people chose to ignore most of the time in the start. Later, when they realize the difference and fail to cope up with it, they fall apart.