Why doesn't SSAS cache the entire cube?

Would you expect a 36GB server dedicated to SSAS to eventually cache an entire 11GB cube in memory? If so, you would be wrong. SSAS does not necessarily keep aggregations in memory, even though memory used is less than MemoryLowMemoryLimit.

I have seen this effect on many production servers but never quite believed it. There is much more RAM than cube size, yet SSAS is continually losing data from cache and asking for physical IO. Actually, SSAS relies on the Windows File Cache to reduce much of its IO. However, the Windows File Cache does not cache the entire cube either.