It's because Ryan (through Homelander) is the result of experiments by Vought.
Ryan is unique, being the only known natural-born supe in the world of The Boys. In Season 3 Episode 7, it was revealed that Ryan's biological father, Homelander, is the result of an experiment by Vought on Soldier Boy's genetic material. Homelander is Soldier Boy's biological son. In addition to being the result of an experiment on Soldier Boy's genetic material, Homelander was subject to further experiments by Vought since his early childhood.
Ryan is the offspring of Homelander, a Vought lab rat, who is in turn, the result of an experiment by Vought on Soldier Boy's genetic material. The unique circumstances of his lineage made Ryan the first and only natural-born supe.
Related dialogue (emphasis mine):
Soldier Boy (to Homelander): Fall 1980, I get called into Vogelbaum's lab for an experiment, some shit about genetics.
I beat my meat into a cup. Turns out... Vogelbaum made a kid. Born spring 1981.
You know what the bitch of it is? If they'd have just kept me around, I'd have let you take the spotlight. What father wouldn't want that for his son?
- The Boys S03E07 "Here Comes a Candle to Light You to Bed"
Another answer states that the premise in OP's question is unclear because of a small sample size issue; OP has only mentioned two examples. I want to explain why this is not a sample size issue.
Aside from Victoria Neuman, the supes: Translucent, Mesmer, Stormfront, Nubian Prince, and Nubia were also revealed to have offspring. None were shown to be supes. Also, and more importantly, Stormfront has remarked that Ryan is the first natural-born supe in S02E07 "Butcher, Baker, Candlestick Maker". This remark is significant, given that Stormfront is the first person to become a supe, and is over a hundred years old. In her entire life, she had not known of any other natural-born supe apart from Ryan. It is clear that Ryan is the first and only natural-born supe.