Ciri is already OP. Why go through the painful Trial_of_the_Grasses and be infertile. Geralt and Yen spent quite a lot just to have a child and it’s hard to imagine Ciri chose the trial.
Also Ciri undoubtedly has trauma surrounding fertility and pregnancy. In the books she was repeatedly captured by people who only wanted her in order to give birth to another child of the elder blood.
Thinking on it, they could further justify it by having ciri decide to become infertile to avoid others from trying to capture her and force her to conceive a child of the elder blood.