And I would say the advice of "just eat less" doesn't work for probably 20-30% of people. Especially the very obese. You don't get to morbidly obese by "eating a little bit too much for 10 years." You don't fix it by eating a little bit less.
Obviously people can starve. There are 3 phases of starvation. Phase I is basically just "between meals" where the liver puts out some glycogen into your blood stream to maintain stable blood glucose.
Phase II is your body using your fat storage to provide energy via free fatty acids and ketone bodies. This can last, depending on the deficit and how much body fat you have, for weeks at a time. The record I've seen is over 400 days of a Scottish man going from over 400lbs to less than 200lbs.
Phase III starvation is when the body begins scavenging lean body mass for energy. This is Very Bad. This is why the POWs you see in photos look so emaciated and skinny. The body used all the fat and then it began using the protein.
The problem for dieting is that you can't just hang out in Phase II until you have six-pack abs. If your body cannot meet the energy requirements necessary from body fat then it'll go straight into Phase III. Many obese people have a "broken" metabolism in some way (how so is not very clear, that's what I'm trying to figure out). Despite carrying lots and lots of body fat around (I had over 100lbs of it on me!) they somehow couldn't "access" that energy. So the body went straight into Phase III with all the signals like reduced expenditure, lower body temperature, lower immune system, increased hunger.. and even if you could willpower though that for 100lbs, you'd lose a ton of protein as the body would use it for energy. So you'd probably die of organ failure before you lost the 100lbs. Unless you somehow managed to do what the Scottish man did. I've fasted for 10 days and I could absolutely not do 400 days.
Bodybuilders are a completely different topic than obese people, I think. They're typically already at a healthy fat level or below. So they really do have to force themselves to undereat and just minimize the muscle scavenging and maximize fat used for energy. But it's temporary for most, they do this in cycles before shows and photo shoots. No bodybuilder I know of had 100lbs of fat on him and then lost it for a show. They go from their walking-around-at 10-15% of body fat to 5-8%.