It's all about control. Also the type of man a woman leaves because the relationship is poor usually sees the DC as an extension of the mother. Relationship over = DC nothing to do with him.
For the first 6 months after we split, my ex paid half of DC2 nursery fees, plus £300 maintenance. £300 maintenance for 2 DC, which was the same amount he pays his ex for their 1 DC. I repeatedly told him I'd be happy to accept £600 for the 2 DC in a private arrangement between us, so it was equal for all his DC, but he refused saying as my 2 DC live together, they cost less(!).
I took him to CMS, and initially he told them he was self employed, so they assessed him as due to pay zero! I sent them evidence of his having been in the same PAYE job for the last 15 years, and lo and behold, they reassessed him as having to pay just over £600 (exactly what I'd originally asked for privately!). A quick call to HMRC would have confirmed that for CMS, but I digress.
He has only had DC sporadically, yet was surprised I'd told the CMS this; in the first year we'd split, he had the DC to stay with him 35 nights out of 365 
Now he refuses to have the DC any more than a weekend here and there, and no extra in the holidays as "that's what you told CMS".
I can already see that when our DC get older and want nothing to do with him he will blame me for poisoning their minds. With no ability to self-reflect, and little emotional intelligence, he won't be able to see that it's his actions of seeing them as little as possible, therefore not investing time in their relationship, is what will get him there.