Initially there is a huge investment of time, costs for hosting if that applies promotion and the build-up to a steady audience of traffic to your blog or site. Advertisers are not interested in a site that does not traffic. Building-up traffic and trust and being seen as an expert in your field is a huge investment. Advertisers appreciate this and are willing to pay to tap into to this traffic.
It takes an enormous amount of time to research, academically underpin proof read, publish and finally promote a content rich blog post / article.
I do not appreciate the view by some who say that if you rely on ad income them you must go if ads are blocked.
Since the dawn of time, we have relied on advertising.
If you think the big giants like, Google, Facebook, Twitter or even Youtube are free, then think again, they all rely on advertising revenue to get by.