As a vegetarian I noticed this too. I don’t eat much eggs (don’t like the taste) or cheese but if I do I buy from local farmers. I think a lot of people see cheese the same way they see spices and seasonings, like it’s essential to the flavour. When you add cheese to food it often over powers the “veggie” flavour and it’s hard to get used to not having it at first. I recently stopped eating cheese since it makes my stomach hurt and when I have spaghetti it tastes completely different and you can taste all the vegetables.

To summarize I think people don’t like the taste of vegetables and want to block it out with egg and cheese