Because a fully vegan/vego diet is not trivially accessible for me. I rarely prepare meals at home, and eat out/order in a lot, and the effort of moving to a fully vegan/vego diet would be non trivial.
A vegetarian diet is something I would quite happily adopt if it was more readily available, but it's not trivial, and it's not a high enough priority issue for me to be able to sustain a non trivial solution over the long term