can the site not auto redirect the main page to the advertisement in some time without the need for user clicking on the link or opening the new tab?
They lose your focus. You got redirected to the ad site, you probably will not get back. If you got interested, you will interact with the target site, not theirs. If you aren’t interested, you close the tab.
can the site not open the advertisement in the new tab automatically?
They can open a new tab and not be blocked only in a short time frame after loading the page, or when you interact with the site. That’s why some sites that opens a popup have a link saying Click here to open, so if your browser blocked the automatic popup, the link will open it.
can the site not open the advertisement in the new tab instead of opening the content in the new tab and opening the advertisement in the main page?
They aren’t interested on the ad being seem. No matter if you saw it ow not, they are getting paid anyway. Opening their site on a new tab makes the history clean, and you probably will close the ad on the original tab, and lose the track of the sites you went until reaching theirs.