Because it's a product in the skinsuit of a beloved story, literally no other reason. No point torturing your head.

The MC has to be Ciri because of member berries, feminism, and the alternative being a white male in 2024. The MC in a Witcher game has to be a witcher or else people get mad plus half the lore and gameplay mechanics are gone, so she has to have done the Trial of Grasses. Any further fake explanation of 'why' is going to be totally post hoc, in fact consider it subject to constant internal revision right up until release.