This question presumes that White Walkers were in hibernation. Were they really? What's the citation?
Why exactly The Night King made humans believe that White Walkers were extinct?
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We know a couple things about the state of White Walkers and their army of wights:
So while there is no direct citation the conclusion we make as readers or watchers is that at least the wights have been dormant and probably in their form stasis awaiting something.
The White Walkers on the other hand been active for at least a few years or decades. We know this because they have been taking Craster's sons and while don't know exactly how old he is, he is described as nearing the end of his life. Craster has always given his sons to "the gods" and has been sexually active for quite a while. We know from the show these babies are turned by the Night King into White Walkers.
So we know the White Walkers are up to something. That something has to do with taking over the human world. However, the why is never explained in the show and the books have not gotten that far yet.
As to wanting the humans to believe they are extinct, well that is obvious... the element of surprise. If your enemy isn't preparing for your attack you have a have huge advantage.