Totally understand your point, and the veg lifestyle is becoming much more mainstream in athletics. e.g. check out Nimai Delgado's (IFBB bodybuilder) instagram posts on simple, high protein, high calorie foods for bodybuilding: https://www.instagram.com/veganbodybuildingfood
I'm not trying to say that's what the people giving advice actually thought; I'm just saying that's how I felt that it came across sometimes.