It's pretty rare that step parents and step children get along.. so I'd say often it's a delusion when folks are marrying 'for the sake of the children'.
How many stepparent/stepchild relationships do you know of where a divorce isn't involved? In other words, how many of those "second marriages" were legitimate?
Let's put it this way: the best thing "for the children" is to put in a heroic effort to get along with your spouse and stay together. But this doesn't apply in the case of the death of one of the spouses.
I think your observation might hold some truth, but it's probably coming from anecdotal evidence from X number of WORLDLY (non-Catholic) families.
As someone pointed out, there is an implicit rejection involved in divorce. Not just the spouse, but the children you had with that spouse. It's hard to believe your dad truly loves you, when he left your mom for another woman. After all, a child is 50% each parent.
When the father/mother of the stepchildren is still living, there is a potential rivalry or jealousy element. They represent your new husband/wife's previous relationship -- a relationship that you'd rather not think too much about.
It's different when that previous spouse has passed on to the next life. No one is jealous of a dead person. There is a fundamental difference.