Honestly I think that meat is a huge benefit towards foods that are essentially cheap and low skill to make. As soon as you remove the meat element, your costs and prep effort start to go up considerably. So essentially, to give up meat, you need to prepare yourself for a more expensive diet that requires a lot more effort to maintain. Ok, well that and the fact that people who grew up with certain foods are going to be heavily biased towards them.
Hm, maybe. I mean I like to cook quite a bit so I'm not really the best to ask if it's easier, but there's also vegetarian junk food. Pizzas, fries, falafel, etc. As for expensive, I think here it ends up being quite a bit cheaper. Fast food isn't cheap the same way it is in the states (a mcd's menu will set you back ~8 euro, I can cook for around 2 euro a person) so that might be a contributing factor. For example (thai) curries are ridiculously easy to make and still be cheap & tasty, you just throw in some vegetables, coconut milk, spices & curry paste, something protein-y and you're set. In my case it mostly required a different way of looking at food, and I've heard this from a few people who became vegetarian after they learned cooking with mostly meat -- instead of thinking "i'd like chicken with..." or "beef with" you have a different starting point. Not hard to change, just different.
Humans are not vegans by nature. If we were,we would have a gut like a gorilla (basically a fermentation vat) and a different gut biome. Our guts would also be way longer.