“There’s a lot of protein in plant material. A person won’t be lacking for protein.”
Sure, but you have to consume a lot of plants, and the proper variety, to get enough protein. By contrast, just a bit of meat a day will fulfill most of your needs.
There’s a reason that herbivores spends all day eating and predators can just do what they need to in order to score a meal, then laze around in the sun the rest of the day.
It depends on what you’re eating, just like anything.
Nuts, for example, are full of energy. You can easily get a full days worth of calories from two lunch baggies full of nuts.
Of course, you need more than nuts to subsist, but I’m only making the point that a vegetarian diet does not mean that it will lack useful calories.
I’m not a vegetarian, for the record, but I am aware that you can get a healthy diet as one if you’re willing to educate yourself.