Why am I extemely underweight?

Hello, I am 20 years old, 5'11 and about 120 pounds (hover a pound or two below or above 120). My ribs still show and like the bottom parts of my rib cage, (left and right) are visible and if someone were to touch me around that area, chest or lower chest, they can easily feel that bone. Other than my ribs I'm pretty boney all around. I don't understand why I'm so underweight I have a normal, healthy diet. I would say I eat much more than a lot of people that weigh more than me and I just don't understand. Is it a super high metabolic rate or something? Intestinal parasite? What can I possibly do to gain weight? (mind you it literally seems like an impossible fantasy to gain any weight for me no matter what I do) Any advice other than the obvious eat more or work out? At this point eating an excess amount of food just seems wasteful because it seems like my body just devours and shows no results whatsoever. I eat normal sized meals but throughout the day I am CONSTANTLY hungry, like I need to be constantly snacking throughout the day in order to satisfy my hunger. This is just getting very frustrating for me now because I always thought as I get older I would get better but I'm 20 and nothing has changed, I have actually gone down like 5 pounds since last year and I dont understand why. Please help. I am very very desperate to gain weight at this point

Edit - I feel I'm also big boned as other skinny people I know still don't show as much as I do e.g. on the wrist and such so that isn't a great combination