It's a lot harder now than it used to be. There's a lot more competition and I have the impression that (at least in my niche) businesses are spending less on advertising and that Google is probably giving publishers less than they used to do.
About ten years ago I created a website as a hobby, it started getting thousands of hits per day and after I started to monetize it I was able to pull in a nice four figures per month at its peak with AdSense and some other CPC and CPM ad sources. Unfortunately, EPC started dropping like a tank when the housing crisis arrived in 2007, and later Google search updates more than halved my traffic. As a result my online earnings are now less than 1/10th of what they used to be.