Because ‘protecting the children’ is really effective, as most people want to protect children, especially their own.

Here’s how I think it goes for a lot of people:

• never have actually interacted with trans people

• the first time you are introduced to trans people is a transphobic friend or pastor or podcast host or whoever. I believe it’s most effective if the person is someone you trust, especially in a position of authority.

• they tell you trans people are evil wicked perverts, and since you trust this person and have never heard of this before, you believe it. And besides, who doesn’t want to protect children? Evil wicked perverts, that’s who, and you are not an evil wicked pervert are you? (Personally, I theorise that people are most likely to believe and keep believing whoever happens to be the first person to tell them about a certain subject.)

• chances are, this leads to you isolating yourself from anyone who could prove you this isn’t true. If you somehow became friends with a trans person, you might understand that maybe they are not evil wicked perverts, but since your first reaction to learning someone is trans would be aggression, this is very unlikely to happen. (You could compare this to how cults isolate their members from people outside the cult to prevent them from encountering different perspectives that could change their mind.)

So, long story short: propaganda.