Why don't people eat things with very high levels of cellulose

Humans do not eat very high levels of cellulose because they are not able to digest cellulose as well as herbivores, eg. cows
Explanation:
Humans can not digest cellulose because they do not have enzymes needed for cellulose digestion in their gut.
For example, cows have symbiotic anaerobic bacteria (e.g. Cellulomonas) in the flora of the rumen [No. 1 on the picture bellow]. Symbiotic anaerobic bacteria in rumen produce enzymes called cellulases that help them to digest cellulose. When you add foods that posess cellulose in your diet [dietary celluose] this is a good step for your diet.
Dietary fibers are good for preventing constipation, reduce consumption of high calorie foods, reduce body cholesterol...
Eating dietary fibers is associated with lower risk of colorectal cancer.
A really good example of a dietary food is oat bran. When you eat it you feel full, really fast, and you eat less in between meals.