There is not a single reason or nutrient to why we need to eat flesh at all since all nutrients have a source which is not from an animal, animals are consumers and not producers. 1.4 billion vegetarians and estimated 15 million vegans worldwide sends a very clear message, we do not require flesh. Now the common reasons to why some people have a hard time switching over to a vegan diet is mostly due to the amount of fiber and existing health issues and the lack of knowledge of how to eat properly. Even some people that are eating flesh lack the proper knowledge of how to eat properly since some people know nothing about real nutrition.
Now if we look to our paleolithic ancestors and their eating habits then they consumed more than 100g of fiber per day while today it's good if we even reach 20g of fiber where people on vegan diets can consume up to 60g of fiber. According to a 1999-2000 study of the US population then zero percent of the people between age 14-50 consumed the daily requirements of fiber and only 3% succeeded if you include all ages. What that says is that most if not all people are prone to digestive issues due to a lack of fiber and that are in many cases hidden until people go fully plant based.
If you are into nutrition then you should know that flesh is inflammatory because of many reasons which include Neu5gc, TMAO, heme-iron, saturated fat, cholesterol and the animal protein itself, if you consume flesh 3 times per day 7 days per week then you gonna build up chronic inflammation in your digestive system and body. Is it then any wonder why so many flesh eaters got IBS and Chron's disease, food intolerance's including other common diseases that are linked to increased inflammation like cancer, heart disease and arthritis?
That said, we can ask ourselves this question. Is there a valid reason to why we need to filter the nutrients through another body when the effects of it is clearly detrimental to our health? Remember, most diseases are related to gut health and what is good for the gut is also good for the mind. Depression has also been linked to inflammation which is why people on vegan diets also are less prone to depression since a vegan diet are very rich in anti-oxidants and contains in average 64x the anti-oxidants compared to flesh. That explains why some people who go on vegan diets also reverse their IBS, Chron's, arthritis, heart disease, cancer and depression.
Just remember, if you spent years damaging your body then you can't expect it to heal overnight, it will take time but as long as you don't stand in the way of your own body then it will cleanse and heal itself because that is one of it's primary functions. If you are a diabetic or have IBS, Chron's or any serious health condition then I strongly recommend you contact a doctor that also understands nutrition who can supervise during your transition.
I end with this inspirational story of how a person with MS managed to reverse the disease with a plant based diet that included meditation.
"What would you do if you lost your vision? What would you do if you completely lost the use of your hands, arms and legs? What if you suddenly became incontinent? What if you were diagnosed with an incurable disease? Well this is the extraordinary story of the musician Bob Cafaro...."
An Evolutionary Perspective Enhances Understanding of Human Nutritional Requirements
https://academic.oup.com/jn/article/126/6/1732/4724763
Do Vegetarians Get Enough Protein?
https://nutritionfacts.org/video/do-vegetarians-get-enough-protein/
Anti-Inflammatory Diet for Depression
https://nutritionfacts.org/video/anti-inflammatory-diet-for-depression/
An Estimate of the Number of Vegetarians in the World
https://www.esri.ie/pubs/WP340.pdf
How Many Vegans Are There In The World?
The total antioxidant content of more than 3100 foods, beverages, spices, herbs and supplements used worldwide
https://sci-hub.tw/https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2841576/