As a general market trend, why are click through rates decreasing? Is it because of the availability of information without going to a site?
Because the advertising industry has structurally educated users to try their hardest to ignore the ads, resulting in every more obnoxious ads, which are then even more forcefully ignored. And that in turn gave rise to adblockers, which are now more or less a requirement if you don't want to lose your sanity while browsing the web.
For a long time I wondered why people said this. I don't use ad blockers and didn't feel it was that bad.
Then 2 things made me understand. First, I pay for YouTube. If you don't and don't block their ads, they seriously test your patience as you browse. I tried it for less than an hour before I couldn't take it anymore.
The second was looking for torrents and hacks (it was for a legitimate and unambiguously legal purpose too, no gray area, but long story!). Those sites are literally impossible to use without ad blockers. Same thing for tools related to diagnosing PC issues. It's all ads over ads over scams and trying to get you to install some adware as you navigate the site trying to install the actually legitimate tools.
I very rarely do either of those things (YouTube without subscription, and navigating the "gray" web), so I never realized just how fucking awful it can get.
It's almost like a sort of reverse network effect. The bigger you get, the less desirable your product becomes. And the advertising industry's getting quite large.
If I want to advertise my new Windex(tm) flavored Mountain Dew[1] then all I need is people to see it. They're not likely to buy it from a click, but I want them to remember it next time they are at the grocery store, and I'll pay to shove it in their browser.
[1] Not currently available in retail stores