Most people don't divorce unless they really feel there is no other option. People don't like change and disruption, let alone the often significant change in living standards that come with a divorce, particularly for women.
The OP refers to boredom, or being "stuck in a rut." IMO when you start probing what someone means when they say that you'll find they've condensed years of resentment into that phrase, and the apparent indifference is more a reflection on the fact they've given up on being able to achieve change.
Take a typical example used by the OP of the couple who have had children and things have gone stale. When the woman files for divorce in that scenario IME it follows years of being left to do the drudge work, the majority of the cooking, the laundry, the ironing, the cleaning, the organising, taking time out to go to Drs with said children, packing the pack lunches, working out what needs to be on the shopping list (even if H actually does the shopping or the cooking)... etc. It's all consuming and leaves little time for W to actually be a person in her own right even assuming she has the energy to actually desire it.
...Meanwhile, H might have a hobby he prioritises, or goes to the gym regularly. It's a recipe for disaster as couples grow apart. And yet, if W says she's not prepared to do that anymore she'll often be told - (and often by other women) - that's she's overreacting to petty housework stuff, or that her H is actually brilliant because he cooks 4x a week and goes shopping (even though she is doing way more because of the laundry and the working out of what needs to go on the shopping list etc). But without that change from the H, who actually needs to pull his weight the full 50% so his W can 'get a life' and each remains interesting to each other, the marriage inevitably falls down.
And as for infidelity. Well, I don't think it's a stretch to say that the sort of person who can blatantly lie to and deceive their partner, chooses sex over time spent with their partner or children, and deflects family money for their own selfish gains, is probably the sort of person who is generally very selfish generally. Sometimes good people do stupid things and a marriage can be saved. Other times, the sort of character flaw that leads to infidelity is a character flaw that is damaging to the family unit (and therefore parenting quality) and divorce is the only sane solution.
IMO divorce is not the problem. IMO the main problem is that many people don't hold themselves or their partner to a high enough standard and be sure of that before getting married or having children. And when I say that, I don't mean to imply that I think 'many' people are stupid, because they aren't. What I mean is that we need to address the many social norms that encourage unhealthy relationships and normalise them.