You made me check and GvA actually only did the Roubaix-Liege combo 3 times, so even for him it wasn't that common.Probably true. I just don't see the harm in trying, I guess. Greg used to ride all classics through Liège, and I don't think it was fatigue that prevented him from winning that one.
I think because Van der Poel did CX his season is effectively a little longer than that of most competitors, so he may just feel a slightly longer break is more important. Also the WC is after the Tour this year, so that's all the more reason to get that Tour prep exactly right this time.