The ceramic packages the IC's come in are porous, so any water used may not fully dry by the time you switch it on.
Ceramic? Intel has been using an organic substrate for their chips since the day they switched to flip-chip packaging over 12 years ago. AMD has not used ceramic in a similarly long time either.
Most modern ICs are packaged in plastic or epoxy which are much cheaper and allow much smaller packaging than ceramic.
The slow warm-up in the reflow soldering process plays much more important roles than baking moisture: preventing excessive warping and stresses due to uneven heating and deep pre-heating the whole board and components to reduce the amount of heat that needs to be applied to achieve the full reflow temperature for the 10-30 seconds that it needs to be applied. The moisture bake-off is a convenient side-effect.