Because I am not, I only almost am, but I eat honey, and sometime I might eat some dairy or eggs if it is in something I want to eat. I drink wine that probably is often not vegan too.
As well, because if you say you are vegan then a lot of non-vegan people won't invite you to dinner or whatever in case you are the angry kind of vegan who will analyze what they eat and try to teach them when you are supposed to be having fun together.
And if you say you are a vegan loudly and do or eat one thing some time that breaks those rules then you get some of those more angry vegans telling you are not and how you are terrible and I do not need that in my life.
I am happy to eat what I do, and not call myself this label 😄