For me, a big part of it is wanting to minimize animal suffering. As for the environment:
The actions of the many do not take away from what needs to be done.
“A vegan diet is probably the single biggest way to reduce your impact on planet Earth, not just greenhouse gases, but global acidification, eutrophication, land use and water use,” said Joseph Poore, at the University of Oxford, UK, who led the research. “It is far bigger than cutting down on your flights or buying an electric car,” he said, as these only cut greenhouse gas emissions.”
https://www.science.org/doi/10.1126/science.aaq0216
There is clear empirical evidence that what we're currently doing is not sustainable for the world for the long term. Just one person consuming one piece of meat like beef would require years and lbs of food and water to raise just one cow. It's a highly inefficient food system when we have other options that can already replicate or even come close to the taste and texture of meat-based products. We will not be able to sustain our growing population if we keep doing what most people currently are doing.
If the world adopted a plant-based diet we would reduce global agricultural land use from 4 to 1 billion hectares.
https://ourworldindata.org/land-use-diets