It depends on the definition of Veganism.
There’s is a popular school of thought that the diet‘s sole purpose to reduce suffering. If a living thing has no central nervous system (or brain), it has no thoughts and cannot experience pain or harm. It’s not much different than a fruit or vegetable. I know vegans that make exceptions for oysters - for example.
Others schools of thought are about avoiding animal products altogether, it doesn’t matter if it suffers or not - there’s no way to know. Therefore, it’s immoral to eat them if you can knowingly choose an alternative.
Suffering is a broad definition. One would argue that prematurely ending sentient life without their consent would fit that definition.
Often, it’s not suffering on an individual level - but the suffering of a species. Cows live in bondage and we benefit from their labor and chose to end their lives for our benefit.
Sometimes Vegans extend this philosophy to pets and service animals - even if they’re treated exceptionally well.
The point is that Veganism is less monolithic than folks tend to believe. A person’s diet can be deeply personal and it’s up to them to draw lines.
I’m a meat eater. I don’t have an issue eating cows. I don’t have an issue eating rabbits, which I know people also keep as pets. I don’t have an issue eating lobster - whereas they’re boiled alive. But I know I couldn’t eat a cat or dog. Realistically, I have trouble with veal. So ya know, where I draw the line might not be the same for other people. My diet is informed by my culture, health, experiences and personal feelings - as is everyone’s.