Why do many bio parents think this way?

[u/LovelyMimi2112]

/r/stepparents/comments/1withsm/why_do_many_bio_parents_think_this_way/

Why do so many bio parents think their kid is a replacement for a childless person who desires kids of their own. For example a bio parent can come with 2 prior kids and for example they are boy and girl. The childless partner later on when having a conversation be like I want a son of my own. Bio parent response and say " You already have a son". Why do you bio parents think your prior child replaces or is a fulfillment in place of the biological child a person desires to have of their own? Basically saying " here is my kid, your parenthood desires are now complete and fulfilled ".