Because the valve that releases "chyme" to the small intestine's "job" is not to pass anything that could give rise to a possible bowel obstruction. You can make a liquid diet just as interesting, tasty, and containing all necessary components, you just have change the way you prepare your food, and it can be monotonous. If stomach accident and enzymes are low, for any reason, digestion will be slower, even though your food is more finely divided. I find the best thing to "moving" the stomach contents and the alimentary canal in general is to keep moving, and that's not easy for some of us, as the more we advance the more we tend to just sit in a chair all day. Drugs are available to help with this problem.