I agree with everything you said there. I believe it's a fact that people don't know themselves and not regarding only the marriage but in every other aspect as well. I think majority of people is not even interested digging up and trying to see who exactly you are etc. For the same reason people are getting in the wrong uni, get the wrong (bad) job, etc. I don't know how it looked in the past but from what I've notice it seems that people are barely mature these days. They all want to enjoy themselves much and hardly anyone wants to invest time in the relationship and to build trust and something for the future. The temper is crazy.
However, I do believe that most (long lasting) relationships and marriages vanish because some third party appeared. So yeah, from what I can tell the divorce rate is insanely high and yeah, majority of the crashes is due to cheating OR simply because they've fallen in love with somebody else, being totally neutral to the partner etc. From my experience it's less than 10% of those long lasting relationships and marriages that crash due to other reasons.