I agree. I remember seeing many of these slides when I lived in Cali...
However, just to play devil's advocate, if you wanted to build on the side of a mountain, couldn't you build a proper foundation (such as pilings driven down to bedrock) to support the house through the mudslides?
I realize that the foundation would cost more than the house...
I don’t think that’s possible given the sheer force and volume of material at play. Even if it were, it’d be a stilt house in the end with no access.