Because being adult in age isn’t at all the same thing as being adult in maturity plus often people derive different benefits in different kinds of relationships so they want to keep both going (for example, somebody in his or her 40s happilly married with kids and yet getting excitement and sexual pleasure with a younger lover).
Also there are often huge social and financial implications to breaking up certain types of relationships or under certain conditions, but people might still be irresistibly drawn to something else, so they play a game to both sides not wanting the costs nor willing to run the risk of losing one to get the other.
It’s a mix of selfishness, immaturity, cowardice and people changing over time and discovering that the stable relationship they’re in doesn’t fit them (either anymore or they get to a point were they figure out it never trully did)