It's just a lot easier and probably cheaper for the company, it's free food, so why all the moaning?
Vegan and vegetarian buffets mean everyone without an allergy can eat something - omnivores, pescatarians, halal diets, kosher diets, vegetarian by religion, vegetarian by other choice, vegan...
Absolutely everyone. No special labels, no hassle when the nice vegetarian thing is scoffed by all the carnivores before the veggies get there, no disrespecting people's religious practices.
Those with allergies who trust corporate catering... well, I guess I salute your misplaced faith courage, as cross contamination is rife. There are just as likely to be allergens in an omnivorous buffet as a vegetarian one.
If you are too distressed by the concept of food without meat for one lunchtime, by all means hoover up the potato wedges/baked potatoes/spring rolls/samosas and fantasise about the half a cow you'll treat yourself to for dinner.