Because without her being a witcher, the gameplay and character building becomes quite different. The whole part about preparing to slay monsters by buffing yourself with potions or decoctions goes out the window. No more mutagens, no more building a sign or melee builds using different mutagens, etc. That's why I said Ciri as a protagonist could still work, but they'd have to revamp the combat and character building parts. Things they were obviously too lazy to bother doing. I thought they would have made her something more akin to a spellsword using signs, basic magic, and her elder blood powers to fight, but no. The first female witcher in history it is.