Why do absentee parents return and expect to be accepted?

Hi. My situation is complicated, but please bare with me. I am 19 and in my 2nd year of university. I was born to a young soldier in the Army and her married Officer lover. My mother didn't know until after she became pregnant that he was married and her being so naive kept trying to make happy home with him even going so far as to talk a nurse into giving me his surname when he refused to sign my Birth Certificate. She experienced Hell trying to keep this selfish man around for my early years. He treated her like maid when he came to visit and bullied her into seeing no other men.He also demanded she drive 800 miles away to bring me for visits that he refused to help fund. She filed for Child Support twice, but was bullied into backing off twice. When she filed a 3rd time I was 3 and the case worker told she was through dealing if my mother backed off again, so she saw it to completion. As anyone might have guessed when the mm an could not convince her a 3rd time he stopped visiting. It took until I was 8 to get regular payments. As I got older and asked questions my mother bragged about his military career while my grandmother told it like it was. She said he was married, I would never be accepted, and my mother had not done me any favors by trying to keep him around.
He tracked me down when I was 15 and he was deployed to Afghanistan. He asked for pictures which my mother obediently had made and after he received them we didn't hear from him for like 7 months. Hell broke loose over that year as he told lies, casted blame, and even raved like a lunatic. I was DONE with him. Once I make up my mind about you that's it. If you're blacklisted then your dead to me.
Enter my financially irresponsible mother. I got accepted on a full ride to a university 1,000 miles from hone, but my mother was unemployed and couldn't afford the ticket. Child Support made my father buy it instead., which tipped him off to where I was going to live for the next several years. Something I loathed.
He turned nice again suddenly and tried to have regular contact. I smelled a rat and was generally disinterested. He soon asked for the Child Support case to be closed again. My mother was going to do it naively believing he was a changed man. I told her the charm was fake. When the case was not closed he started raving like a lunatic again. He owes $8,000 in arrears to my mother alone and is barred from both a passport or up to date driver's license. It's been a year since that tirade and he still hasn't paid it off, which tells me he must not care if he can't drive or travel. A social worker called recently and tried to cajole my mother into backing off again. She called my mother selfish and greedy. My mother passed the phone off to me to assure the woman that relationship had no place in this. She asked if I didn't want to know my father.I told her that all I wanted was for my mother to be paid , so I could finally change my surname from his to something I liked and for that man to NEVER contact me again. She had the nerve to tell me I was the rudest child she's ever seen.I told her I meant it and she could tell him I was waiting to dance on his grave. Seriously though, men are not around when their children are growing up, refuse to help care for them, then come back and expect to be accepted? You're dealing with adults not children you can force to like you. Why don't absent parents just stay gone?