1. Development/physical disabilities, or overall health issues
2. Reaction from others, will immediate/extended family help? It's one thing to ask, another for the reality to set in. Overall support base in general.
3. Employment realities. Will a lot of employers have explicitly stated policies, the day-to-day realities often differ (and can be very different between mum(s)/dad(s)).
4. Just general variations in children/growth. I have two, one eats but doesn't sleep well, the other doesn't eat well but sleeps soundly. One enjoys time to himself, but has a temper. The other is far more social (so much social) but is very even-tempered and empathic. No amount of preparation can provide enough knowledge/experience to deal with these qualities, not withstanding 1/2/3.
As others said, we read books, attended classes, looked after nieces/nephews, younger siblings, baby sat etc. We were still woefully unprepared for our own.
So, is it possible that some folks are "fully prepared" for parenthood? Almost certainly. I'd expect that the numbers work out such that people like this do exist. Parents that were fully ready for the children they got.
However, this is like expecting someone can know exactly how to play poker, following the rules on when to bet and when not to, and then getting shocked to see that you still don't win every game. Statistically, you will lose games. Best you can do is have the game setup so that the losses are small and the wins are leveraged.
"You can definitely be prepared for what cohabitation/marriage is like before your first relationship. It may be a lot of work, but it is not beyond comprehension."
Yes, the brain is capable of comprehending life in cohabitation, as proven by the current mental state of all who are currently in the situation. Getting to that mental state without experiencing cohabitation is practically impossible.
Now swap out "romantic partner" with "small person completely dependent upon you for survival for the next 18+ years".