The reason why Christ had no earthly father is thus.
There are two types of sin, original and natural. Original sin is the sin that is imputed to us through Adams sin. Because Adam sinned we all bear his sin, and that sin is imputed to us through our earthly fathers. Natural sin is that which we give to ourselves by not being holy and by violating Gods holiness, sins such as lust, theft, etc.
So, just by being born, we are sinners; assuming we had an earthly father.
Jesus, in order to be the perfect sacrifice for your sins, had to be perfect; no original sin or natural sin. For Jesus to have no original sin, he had to be born with having an earthly father, “the Holy Spirit descended upon Mary.”
For the reason Jesus had to be born at all is thus.
Jesus had to be fully man and fully God in order to satisfy the demand for the perfect sacrifice. In order to fully man He had to be born.
“he had to be born with having an earthly father, “
WITHOUT not With
If Christ had an earthly father, he would have an earthly Nature without a divine Nature and He would not have been able to declare himself to be the Son of God.