Not quite true.

This app makes money because anyone that owns a cat and an iPad looks for apps that their cat(s) can play with.

The same is true for baby apps. There are some super simple apps that babies enjoy, and those apps make very good money considering the development effort.

That being said, not everyone can pull it off. This particular cat app is fairly well done in my opinion. One must also keep in mind that just cause it makes $4k/month right now, it doesn't mean it'll continue to make that much 6 months from now.