I wonder why it is so hard for vegans to accept that Human do need to consume animal products
Because they are in denial. They don't understand the order of things. They would protest a lion eating a zebra. They are idealists who have been raised to believe that all pain and death is bad.
That said, I sympathize with them in many regards. I don't like the idea of anaimals harmed. It's just that getting a swift blow to the head of cattle is much better than the alternative - letting a pride of lions slowly kill it while it shrieks in pain and knows it is going to die.
They will also object to the conditions the cattle are raised in. This is a valid complaint, except that it is too broad.
Someone does need to inform that vegans, that even if humans switched to a complete herbavore diet, death and destruction would still enter the animal kingdom.
Above all else, I am getting to the point in life "Get of my lawn" applies. I can't be shamed socially anymore, I have a mind of my own. A lot of groups operate out of fear and shame... Once you grow out of that, you can decide for yourself what morality you are going to follow, not someone else's. After all, you are the one who will have to live with the decision and the consequences of it, whether it be shunning or prison, if your morality is way, way outside the norm.