Thus did nobody bother to examine his yearbook?
Correct. Which makes sense. If someone is an idiot now, it is unlikely that the person stopped making idiot statements. So the investigators tend to check statements and actions from the last ten years or so (example source). The normal assumption is that if someone was an idiot in high school, that the person grew up prior to being nominated for a judgeship. So why check high school statements?
I'm unsure how universal the United States treatment of yearbooks is. In the US, it is typical for people to write in each other's yearbooks. So to really investigate, they couldn't read just one yearbook. They would want to read the yearbooks of all the classmates, especially the friends. Because that's where they'd find things written by or about Brett Kavanaugh or whomever they were investigating.
They still didn't check the yearbook during the Supreme Court investigation. They only checked it when the last minute allegations arose. At that point, he had already passed the normal investigative process. The current investigation is in addition to that.
In this case, the accusations come from the high school and college years, so people are looking more closely. And because Democrats oppose the nominee, they are looking for any excuse to make him look bad. Politicians have incentives to act this way.
This is why it is preferable to bring up allegations like this early in the process. Then the normal investigation would have uncovered this stuff and generally ignored it unless it had more serious implications. So instead of pointing out that an entitled rich kid was doing entitled rich kid things in high school, it could concentrate on more recent information or more legally relevant information.
Note that as a general rule, the information in Kavanaugh's yearbook doesn't say anything indicating that he attempted to rape Christine Blasey Ford on its face. There are a couple things that can be interpreted as indicating that he wanted to participate in a threesome. The claims here are that he drank too much in high school (and later in college). A typical FBI investigation would focus more on whether he drinks too much now, possibly by looking at behavior in the last few years.
There is some risk that Republicans will start treating Democratic nominees the way that Kavanaugh has been treated. They may start looking in older material for things that are not illegal but simply unseemly. That seems unlikely to make nominees better (unless your definition of better is someone with few close friends in high school), but it could allow them to score political points.
And if Republicans treat future Democrats this way, Democrats will certainly respond in kind.