Reason for this is simple. Before you can go and boldly Quit your day job to solely focus on your Hubs, Blogs, And Websites you have to be established both in creating good Hubs and allowing for a couple of months for traffic to build up for them.
You can yes, build Hubs Blog And Websites with high paying keywords from Google Adsense ad Units and other Publisher Programs but as from experience nothing is over night and everything takes time. I've built over 30 hubs so far and been @ this for over a couple of months now working on my Hubs while working my day job and have seen little improvement day by day.
So in closing, make hubs with good quality write until your heart gives out and over time you'll see traffic good click through rates and maybe just maybe one day you can get up look your boss in the eye and say "I Quit" I know for myself I dream about that day when I can solely live off my Google Adsense and other publisher programs I am apart of.
I wish you the best of luck