For the simple reason that no digestion takes place in the human intestines. Digestion takes place in the stomach. Intestines are where the nutrients are absorbed into the body, and the conditions there are much more favourable for parasites like tapeworms. The stomach produces hydrochloric acid strong enough to burn a hole in a rug and is also home to lot of other strong digestive enzymes. Any tapeworm venturing there would get digested in seconds.
