Excluding very temporary arrangements, which I understand could be the right choice even if difficult, I can't understand how anyone could do it. And I don't mean moving between neighbouring countries with land borders so that it might not actually be that far.
My friend and I are both immigrants. We both have children with natives of the country we moved to. My friend left the mother of his child a few months after he was born, but so far has been co-parenting his son quite well with his ex. The boy is now 10 and my friend is making noises about moving back to his home country (3 hour flight away) because he's just so homesick and he can't bear it any more. He is a citizen here, speaks the language, has a good job and a wife who is also a native of this country, so he's well 'integrated' if you want to call it that. His ex is obviously going to be staying here with his son no matter what he decides.
I just couldn't imagine even considering moving away from my children. DH and I are still together so it's different, but when I chose to have children in this country with a native of this country I knew that I could never move away unless we all went together. I committed to that and if we ever did split, I'd be staying here no matter what because I couldn't live in a different country to my children, at least before they reach adulthood. And my friend's son was planned but even if he wasn't, when you're a parent you have responsibilities.
I said to him something along the lines of yes, that must be difficult, but there's not really anything you can do about it because of your son, but he just sort of uhmed and ahed about it.
Am I being unreasonably judgmental to think that a parent shouldn't even have this option on the table, or at least not for something like homesickness? In my experience as an immigrant, homesickness can come in waves occasionally, but it passes, and it's usually a case of rose-tinted spectacles anyway. I can understand wanting to be closer to parents, siblings etc. but not at the expense of being so much further from your actual child.