Hi vegan here, stop talking shit about something you know nothing about. Veganism is not living just on air and water.

If there is a cow shitting there is no problem taking that shit and putting it on a field. The problem is when a industry - that is based on exploiting animals - profits of shit flooding our groundwater with nitrate. We want to reduce that huge industry in different ways:

Basic for every vegan: food based on plants not on animals.

how and why people choose to be vegan is as diverse as "being a meateater" is. Some people eat a cheeseburger a day some eat one piece of deer a year.

What I was doing most of my life is environmental veganism: Never bought anything animal related. If e.g. meat was actually thrown away otherwise I'd eat it as well.

Then there are health reasons. (Never digged to deep into that - I am damn healthy and happy with my diet)

And what you are talking about is ethical veganism: Ethical Vegans say there is no reason why humans are allowed to treat animals in that way and think animals are somewhat equal to humans. They would strongly oppose the use of shit of animals on fields if it is possible with less support of the harm the industry does.

Depending on the reason why people do it they often live it in a different way (and sometimes hate each other for their approach)