There are some zealots whose ideas and ideals become so ingrained in them, that it is, quite literally, a complete part of their personal identity and personality. To change them is to all but kill the person, metaphorically speaking, and "reprogram"them altogether into a completely different person.
There are some militant vegans out there who would rather starve to death than eat even a single scrap of meat, even a bug. And then there are those that actually DO starve themselves to death. They call themselves "breatharians" who claim they can survive off of nothing but air and water like plants do. And some who go to an extreme and claim they don't even need water, just air and light.
The reason people don't often hear of these individuals is due to the less than stellar survival rate of those that ACTUALLY attempt it. The others just pretend to do so and video themselves making claims, while eating and drinking "off-camera".
Yeah I heard about the breatharians. They're rather manic hypocrites in my humble opinion.
As for the vegans... I could go off about them for ages. They want to stop using honey and end enslavement of bees - and then use agave and starve bats.
They also claim that ending meat will save the environment, but neglect to think about the transit costs on the environment for all their food coming in from all over the world.
I'll leave it there.