That shelf is in a room private for them, supposedly no one would go for or be interested in the first place
That's like asking someone why they wouldn't know how their great-great-grandparents first met eachother despite their great-great-grandpas diary being in some storage box in the basement that they can access at any time.
But this is the layout of the mansion, it contains useful information. If they need to renovate anything they're gonna need it. There are 3 generations of the Chester family living together in the mansion and no one knows about a whole floor underneath them? Especially when it has escape routes?
Why wouldn't the Chester family know about something like this? The layout of the mansion isn't even hidden properly, it was on a shelf.
I hate myself for taking it this seriously