Why does Peter specifically reference the generation of Noah in 1 Peter 3 when there's been thousands of years of dead people?

18 For Christ also suffered for sins once for all time, the just for the unjust, so that He might bring us to God, having been put to death in the flesh, but made alive in the spirit; 19 in which He also went and made proclamation to the spirits in prison, 20 who once were disobedient when the patience of God kept waiting in the days of Noah, during the construction of the ark, in which a few, that is, eight persons, were brought safely through the water. And then 1 Peter 4:6 6 For the gospel has for this purpose been preached even to those who are dead, that though they are judged in the flesh as men, they may live in the spirit according to the will of God. This passage is about the "harrowing of Hell." There's been at least 6,000 years of dead people, yet Peter specifically limits Jesus' interaction with the dead to the generation of Noah who perished in the flood. What's going on here?