I've launched a few web apps and a mobile app but have yet to make a single penny. I've a few more as well 'in development', which I may get around to releasing

I've launched a few web apps and a mobile app but have yet to make a single penny. I've a few more as well 'in development', which I may get around to releasing some day... Basically everything I have built is because I found it interesting rather than trying to make money, so I'm not too bothered. However if it is so easy, where are all the "Hello World" tutorials for bringing in revenue?