I dont think a person just ups and leaves the kid.
Its more like 7 years in, you find your wife was cheating and then lying to you all this time. You rightfully decide to leave her.
Once you move out, you decide you want to stay in the kids life but yall live in different houses, you get no rights to see the kid by law cause you arent related at all genetically, and even though you make an effort to stay in the kids life, it starts to get tougher and tougher and soon you only talking to the kid during holidays.
Especially when seeing the kid means seeing the mom and you hate that bitch, and even though its her fault she still bitter that you left so that makes coming around harder.
Add to the factor soon you have a new girl and possibly a new kid, ex gets married and theres a new dude in the house.
So many factors come into why in 2-3 years you almost not seeing the kid and its not cause you dont love the kid or your cold. Shit just happens.