If you wrote a blog about leatherworking and wanted some 
    ad money, you could go to a leatherworking tool company 
    and say "I have this much traffic and its 100% people
    interested in your product becaus

It does not work.

For this approach to work, you need somebody who...

1. Is good with the actual subject matter (leatherworking or whatever)

2. Is a strong writer/communicator

3. Has the "salesperson" personality to go and promote their blog to potential companies

4. Has the TIME to promote their blog to potential companies, deliver metrics so they know what kind of exposure they're buying, etc

5. Has the appropriate technical chops to run/publish a blog, handle ads, set up analytics, etc.

Those are disparate skillsets. You are going to exclude an AWFUL lot of good leatherworkers who'd otherwise be good at this. Sure, the occasional unicorns who combine these skillsets exist.

Just like there are a certain number of people in the world who are competitive skateboarders and... are also taekwondo masters or world-class violinists or something.

And if you're lucky maybe 0.1% of that remaining 0.1% will actually have the time to pursue something like the venture you describe, which tends to rule out folks who are already working 40+ hour weeks and have kids and such.

In reality, we're talking about an operation involving several full-time people with a breadth of skillsets to make this kind of operation a sustainable business.

A lot of podcasts and streamers do kinda make this work, but are generally part of networks and in the end are still beholden to advertisers whose businesses are an odd fit with the subject matter at hand anyway -- like all the tech and true-crime podcasts who sell ads to Stamps.com. It's not really the smoothly-integrated seamless bespoke ad paradise you want.