Would anyone in the real world see a cliff and jump off, hoping to meet an old man at the bottom who would save them and teach them peerless martial arts? Getting smashed into meat paste was the only outcome for anyone who jumped off a cliff.
The three Spider-Girls following Cathy Weber had accepted the spiders' gift because they had plenty of living examples around them proving it could work, and pursuers behind them who wanted their lives.
With role models and support ahead of them, and lethal pressure behind them, they had chosen to accept the ritual in Amazon and become part of the Spider Totem. Gwen Stacy had only Henry's empty promises. She had neither the pressure of needing spider powers to break out of a predicament nor any desire to become a superhero.
At the very least, the future she had planned for herself was to study hard, join Oscorp under Professor Kurt Connors, become a highly paid senior researcher, and perhaps even independently lead research projects.
Maybe she would want to find Thena and learn to work out, maintaining a fit physique. But that was entirely different from becoming a superhero.
In the universe of Spider-Gwen, Peter Parker had been sacrificed after turning into the Lizard before she had resolved to put on the white Ghost-Spider suit. That was also why Henry had not actively arranged for the Amazon Spiders to meet Gwen. He simply could not think of any reason to persuade her. No normal person would agree to something like this.
Unless he released the spiders to bite Gwen when she was not paying attention. But then Henry would be making the decision for the girl. That was what he had wanted to avoid from the beginning. He had no right to decide