Movie stars (both male and female), are wealthy and for the most part far more attractive than the average person, so they can afford to 'start over' at any time even if they have 10 kids to support.
True love is really a machination to make life easier for the everyman.
Most average men and women can't afford to be starting up new relationships every 10-15 years, the cost of kids and real estate is too high, and will they be able to find somebody attracted to them when they are decrepit at age 60 (or sooner).
And what pool are you gonna choose from anyway at that age?
So it's just implanted in our heads to stay together for 60 years because it's easier.
Movie stars do not have to do this tho.