Of course it's not all men, but sadly, it's a notable percentage rather than a few outliers.
I've noticed that the crappy ones fall into a couple of camps - degrees of crapness? But what they all have in common is a deep seated belief that the children are not their responsibility.
so a very common one I see a lot around here are the dads who do genuinely love their children, and feel some responsibility towards them, but ultimately see the core job of bringing them up to not be theirs. Men who do the EOW, pay maintenance etc, but just aren't present on a day to day basis and don't really think they need to be. So they're more like kind uncles, than real parents. I find this hard to get my head around, but it's pretty common, and as long as it's done consistently, I think the whole family deals with it okay.
Then there are the ones who come and go, resent paying, can't get their head around the fact that children might not want to do what they enjoy, or might behave differently and so resist spending time with them or only want to do the odd, super fun thing. If you ask these men they'll tell you they love their children, woudl do anything for them, etc etc, but really they're pathetic and useless. These men often justify moving on entirely when they get into a new relationship as being "better for the children" etc etc.
Then of course, there are the ones who just disappear completely. Who don't turn up. Who never pay a cent. Who say awful things like what OP's ex has said to her children.