The first two allegations were, while weak due a lack of corroborating evidence and the fact that all direct witnesses have categorically denied the existence of the events in questions (and in one case denied knowing Kavanaugh at all), at least somewhat plausible. There was a chance that such an event could have evaded the notice of the many background investigations that have been run of Kavanaugh, as unlikely as it seems. This most recent one though, accusing him of attending gang rape parties?
Brett Kavanaugh has undergone six background investigations. At least one of these was an SSBI (Single Scope Background Investigation, done for all clearances above "Secret").
isn't some "Hey we checked a database and you don't have a criminal
record". This is a "We went and talked to your old next door neighbor.
And your friends growing up. And your mom. And that girl you kissed in
the fifth grade. And the dude you bullied in the second grade. And your
father's brother's nephew's cousin's former roommate. And he said you
did weed in the sixth grade, so you don't get a clearance."
The chances that an SSBI would miss Gang Rape parties is beyond ludicrous, to the point where if the accusation is true, we've got bigger problems than who the next SCOTUS justice is.