Logician: When people get married they already know how to treat their spouse from their upbringing, whether in a good way or a bad way.
Sure, chosson & kallah teachers can do more teaching in how to treat a spouse, but a few lessons is usually not going to do the trick for those with issues.
These boys & girls who are going off the derech after they get married, are not ones that were emotionally healthy and erlich and after they got married all of the sudden they wanted out of yiddishkeit due to the hardships in their marriage.