They largely do?

Divorce rates are much higher among people with lower educational attainment

Recently I've noticed that divorce rates amongst the richest are actually quite high too. Gates, Bezos, and Musk are all divorcees and Buffet has had an open marriage situation for something like decades I believe. Just something I found interesting.

They call it "new wife money" for a reason when you get rich...

I think “preach what they practice” would mean publicly speaking (more?) in an effort to nudge behavior more towards how they act (in your example, to speak out in favor of sticking out and improving marriages).