Mold and bacteria thrive in moist, humid environments. It’s less that humidifiers have been engineered to need excessive cleaning and more that their intended function encourages this growth as a natural consequence.

Preventing it would require doing things to the water that you probably don’t want to do to something you’re intending to aerosolize and inhale, or else treating the device itself in ways that are likely to require cleaning and/or maintenance anyway.

So at that point it’s cheaper and safer just to tell people they need to clean their humidifiers because the thing that makes humidifiers good for you also makes it good for stuff you don’t want building up in the humidifier.