Nobody asked anybody to come, and nobody took care, even if seeming little, more then one's parents.
Maybe easier to grasp: Some time ago, even in regard of later relation, a devoted told that his wife gives/gave just out of defilements, greed, into their relation, and because of this, he decided, there is no need of gratitude and he could do as he likes.
He just forgets that his part to it is/was likewise just out of defilements, greed, thinks somehow that he has a right on relaying, maybe thinking his part is a pure.
Again, it's neither required to uphold and maintain relations, if seeing really harmful for oneself or others, but either maintaining, or leaving a relation, requires gratitude. As long as not understand, as long as not repaying the part of goodness, by denying goodness and proper obligation, one is not capable to leave.
to 1: There is a way to repay once parents, yes:
Kataññu Suttas: Gratitude 
to 2: Everybody, not in the Noble domain, nourished and seeks relation, seeks for children, off-springs does such only because of greed, craving, because of ignorance. Samsara cannot be end by "birth-regulation" or by "children/raising-education-systems". They are then just slaves of societies, state slaves, with the same issue like with parents.
to 3: Every being, in every relation, is always free to act liberal or selfish, kind or evil. That is why there are debts, other than if it would be fixed destiny or controlled by a creator God.
to 4: always pure selfish, aside of the eight people, Noble Ones, and only Arahats are real giver, Bhagavato. With finding into the Buddhaparisa, giving into any selfish relation, or after children, has end.
So didn't good
blazer , notout of pure selfishness, demand and nothing but desires, not only take birth by went into what ever other relation?
That being so, the debt thing, works therefore even more binding and unrepayable, once getting in relation with the parents in Dhamma.
Developing gratitude toward oneself and all others, mudita, is very needed, not only for gaining any Jhana (the Brahma viharas), but also to gain path and fruits.
How can one develop mudita, appreciation toward all beings, frim right view, if not capable to feel obligated toward clear giving?
And yes, althought most, even if claiming to be advanced yogis, are incapable of right thinking, even the happiness and gains of most evil seeming people, is required to appreciate, to be able to leave upwardly.
Right view, gratitude, is so unknown in modern world, so denied that it's most difficult to get a real base of Samvega.
There is another of the less teachings in western language, ggod to seek for, read and reflect:
https://sangham.net/en/lib/authors/wijesinghe/gratitudeThere is hardly any modern/western teacher understanding and teaching right view, so understandable that hardly ever to meet the basic of the Dhamma.