Mold is life. Life grows where there is water. Humidifiers dump tons of water in their general vicinity. It's a natural consequence of humidifiers.

As noted by the other person there are ways to get around it by making the humidifier itself out of materials that kill anything that it touches, but those materials are almost never safe.

Copper would probably be the least concerning from a health standpoint, but copper is not cheap, and it's really heavy.

It's way more practical to just ask you to scrub it every once in a while.