It's not just in fantasy that bowel movements are ignored. Excretion is seldom relevant to the plot of a novel — though some writers drag it in by the heels even where it's not relevant.
In Book I of
Gulliver's Travels, which is fantasy by any reasonable definition, an important plot point turns on Gulliver's urination; and in Book III, the School of Political Projectors had discovered how to identify traitors by the properties of their stool. And in
Ancient Evenings, Norman Mailer posited an entire system of magic based on excrement, and fathered it on the ancient Egyptians. Such cases are rare, but probably not much rarer in fantasy than in other categories of fiction.
As for washing in WGW, every time there was a sun of rain, TC and Linden took a shower for two whole days. How much cleaner can you get?

Without the Quest, our lives will be wasted.