because it's just not fair. there will never be greater love than that of parents and this kind of love should never suffer from separation - not even for such a short blink of an eye as human life is. They will meet again and they will meet soon, but there's no time short enough to be excused by any purpose, no separation bearable enough to be understood and accepted.
They will meet again, right now they're already reunited in His Land, for there's no time, holding hands and getting lost in each others eyes, mom seeing her son just like she remembered him, just like he was always there in her heart. And he didn't see the time pass between he left and came back to her arms, for him there wasn't a second without her, yet she lived decades which felt like centuries without him. That's why it's not fair - even with happy ending and all, it 's not fair to let parents suffer this much.