Yeah - I guess it is like everything in the world. Nothing is coming easy. But you can do some good bucks with casual games, I guess. There are several ways to monetize your games. Showing ads is just like one option. You could also sell licenses (exlusiv/non-exclusic), site-locks, premium features, sponsorships etc. to gain money.
In addition, you could also port your game to an mobile app (casual is big in mobile) and sell it through the app stores.
I only released one small casual game on the net and did not make much bucks with it - but I did a lot of research - and as I said in the beginning - there are ppl who make a good living out of casual games.