For safety. Where I am from, magical people are a minority, and historically those without magic have tried to harm us - or anyone else they suspected of being one of us. Secrecy laws were passed to protect those in both societies.
Preya doesn't seem to have those problems, perhaps because the proportion of people with magic and similar abilities here is higher. I wouldn't want to be the only publicly-known witch in Preya, though, in that hypothetical scenario. I doubt it would be very safe.