Why don't celebrity relationships last?

Privacy and pressure to keep people away to keep them safe.

Egos

They can spend long amounts of time away from each other and yes they can get jealous of each other, if one has more success than the other.

Marriage is about trust and if you feel you can't trust your partner the relationship will fail.

In their line of work they are often meeting attractive, charming, gifted people. So there is more temptation.

Many men who are on the road without their partner will cheat because they are lonely. There are however some wives of celebrities who will put up with this, as long as the cheating is discreet and casual.

Will and Jada Smith have been married for a long time but they are rumored to have an 'open marriage'. They both have other partners on the side.

I don't think I could put up with the cheating under any circumstances because it's such lack of respect. Bit quite often the wives know about it and choose to stay regardless.

Same reason most relationships don't last. They are based on the 'idea' of someone or something, rather than someone or something 'real'.

The relationships don't last because they jump into them too fast.

Alot has to do with they don't really see each other very often because they are very busy filming, touring etc.. Unless they are both working at the same job they don't see eachother very much. They go weeks/months without seeing eachother.Celebrities cheat alot. since they literally get everyone throwing themselves at them it can be very tempting for them. There is alot of trust and Jealously issues probably, All the media and paparazzi & fans getting into their personal business. Imagine reading what your spouse is doing in the tabloids or reading some rumor that isn't true that you have to go and ask them.
It can take alot of toll on Celebrities that it can break them apart.

That's why Celebrities shouldn't really get married. about 95% of Hollywood marriages just end in Divorce