While they might be capable of 96, the devices are probably identifying their capabilities to Roon as only 48. Google’s own app probably doesn’t even check, but, Roon does and uses the devices’ responses.
Roon does not have anyway to force a resolution which the hardware itself says it cannot handle.
There are a couple of threads if you search on the forums which go deeper into the technical specifics if you are interested. Here is a good one to start: