@Hamin Not wrong, but I think the explanation is reasonable enough to suspend disbelief.
Namely, the witches are being hunted and suppressed like crazy, so every time they use magic risks exposing them and costing them their lives. If they're out in the wilderness, then it's the problem of constantly being on the move to avoid being caught by hunting parties. Because of that, only witches with very useful powers like Nightingale get the chance to practice.
Also, there's the propaganda aspect to it, but that's hit or miss since they're against the church anyway. Point is that the truth is being actively withheld and twisted.