http://www.paulgraham.com/identity.html
On a side note, I've always wondered why "shellfish vegan" isn't a thing. They don't have brains or central nervous systems, but they do have the nutritional components of animal products.
It's the kind of conclusion philosophers reach, but doesn't seem to resonate with people.
So while I know plenty of vegans with their own particular exceptions they don't talk about them outloud much.
https://news.nationalgeographic.com/news/2013/13/130208-seaf...
A secondary reason is that it's really easy to confuse people who do choose to eat meat about specific dietary concerns, especially when traveling globally. The Japanese for instance put fish into nearly everything and genuinely have a hard time understanding the difference between a vegetarian/vegan/etc there's also been some confusion between daishi and fish. Add to that the number of people who claim to be vegan/vegetarian but don't adhere to it at all and it can make a lot of people see it as a fad diet versus a diet adhered to rather strictly in other people. Another issue is religions that say fish is not meat. This isn't even getting into products people don't realize comes from meat. By just saying I don't eat anything that can feel pain it makes it significantly less likely someone will accidentally feed me meat. My wife on the other hand eats fish/shellfish. Our daughter chooses her own diet and has so far chosen to be vegetarian after deciding she no longer wanted to eat fish. To each their own, everyone has to be comfortable with their own choices.
Likely because of the issue you've pointed out regarding identity. The issue is diet-as-identity. Perhaps it would make sense to have a 95% vegan or vegetarian diet supplemented by the occasional animal product, but you can't claim an identity out of that. Veganism is viewed as some arbitrarily binary lifestyle where either you are or you aren't and there's no middle ground.
Maybe that goes without saying, but that last paragraph seems potentially reductive without qualifying it in that important way:
>But there is a step beyond thinking of yourself as x but tolerating y: not even to consider yourself an x. The more labels you have for yourself, the dumber they make you.
Most of us are going to inevitably be a healthy number of X's (the alternative is not really desirable, as many people along the borderline spectrum can attest to) so thinking of yourself as an X who is tolerant, accepting, and supportive of Y's is still always going to be inevitable and extremely necessary.
I would very highly recommend reading David Foster Wallace's Consider the Lobster.
You can either buy the book of the same name which includes other essays or here's a PDF of the original article: http://www.columbia.edu/~col8/lobsterarticle.pdf and here's a video of DFW reading it out: https://youtu.be/_fZOl7C_vDI
Even though I wouldn't eat their eggs, I can't really see the problem with having chicken around if they don't suffer. For people who eat vegan for ethical reasons, my impression is that suffering is the main concern.
My point was that I find it surprising people with this position who "break" for medical or energy reasons don't generally arrive at the shellfish conclusion. They eat eggs, dairy, meat or fish.
That sort of logic gets you back to farming animals for meat though. We're giving them a relatively cushy life compared to wild animals (at least in some cases), but cutting it short. It's easier to maintain the moral high ground if you simply cut out all animal products period.