It depends on the ad. In the before time, long ago, ads were a simple static image or body of text. And in that case, while not impossible, it's rather hard to compromise a computer with a static image or block of text. You'd have to have a fairly specific image crafted to adversarially target a specific bug in the rendering of images to get code execution, and thus compromise a computer.

It is no longer the before time.

In the now time, ads frequently contain not just text or images, but javascript as well. And already having code execution by virtue of javacript, it is a lot easier to escalate the privileges of that code execution from the limited environment of the browser to installing code on the computer running that browser. Want to deploy your bot? Buy an ad that includes your malicious javascript payload. Now, anyone who goes to a site and views your ad will execute your javascript for free in addition to your offer to sign them up for credit score monitoring.