There's no good scientific basis for "men are from mars, women are from Venus" psychological explanations. The reasons are societal.
My own experience as a very hands-on father is that I sometimes feel I am swimming slightly against the tide. I feel circumstances often .. whispering?.. that I am the back-up parent. All sorts of little things: at my DDs' school, for example, I reckon mention of mums versus dads is probably about a 7:1 ratio. There are the slightly surprised looks from mums at playgroups, and the sense of nervousness around playdates when my DW isn't going to be present. I don't want to say what about the menz, or suggest that men have it better than women, but old societal habits do die very hard.
And also there's work: for most of the last few years I've ended up as the sole breadwinner. We never intended it this way, but even before children I was always more securely employed then DW and, in consequence, she has more time and energy to give the children than I do.
And I also worry that what I do give my children won't really be noticed. My DDad worked like a slave, and gave us a comfortable upbringing, however, my elder siblings take it completely for granted. I worry that my DDs will treat me the same way regardless of what I do.
I think a lot of men have the same perceptions as me. So if the family unit breaks up, the push factors isolating them away from the family group are strong. Perhaps they want it, but perhaps they feel that it is an inevitability.
I will add that I've heard (ref my previous comment) that family courts in the UK will typically award custody to only one parent, something which I have to say is surprising (if true - can anyone enlighten me?). Here in NZ, the default option is joint custody and fathers are expected to do their bit. Also, child support is collected by the flinty-eyed Inland Revenue, so there is more incentive for fathers to share care and by doing so reduce (or eliminate) their assessments, and maintenance orders can't be enforced after 3 years, which puts additional incentive on mothers to work (and not try to exclude fathers from parenting). I reckon all these things reduce the push factors on fathers.
As an aside, DW and I went through a prolonged rough patch, and at one point splitting was a real possiblity. DW assumed that in a separation she'd get sole custody, and was very surprised when I told her that the family courts simply didn't do things that way unless they had to. She also assumed that I'd be the one who left the family home (as both our names are on the title I didn't quite see it that way). I think that attitude is quite common amongst both men and women: the man is expected to bugger off.
I know some older men who have children by previous relationships (before the days of shared care). They didn't see any purpose in trying to carry on being involved with their kids because it all a lot of fuss and bother, and they felt that their kids wouldn't give a shit about them anyway. The impression I get is not that they didn't care, but that they were resigned to it.