This actually isn't that true either. I was a running a lot last year, and eating "better" from April until September, at which point I got Patellar Tendonitis. I was running 18 miles a week, and my diet consisted of 1600-2000 calories, LOTS of fruit/veggies. One day a week, usually Sunday, I'd eat whatever I wanted. Be that pizza, fast food, etc.
I started at 210 pounds in April and got down to 180 in September. After I was told I wouldn't be able to run for 6-12 months while requiring to do physical therapy 4-5 times a week, I stopped giving a shit and started eating whatever I wanted again. From September to March(Now), I gained all my weight back. Five months to lose it and six months to gain it, and I wasn't even being that strict with losing it.
I'm out of my rut now, and can probably start running again shortly. I still have this god forsaken injury, but low mileage isn't going to hurt it. Doctor said I should just increase the days I run if I want to increase my mileage.
TLDR; To answer your question about why it's hard to lose weight, at least for me, eating delicious food is incredibly satisfying. Feeling full is satisfying. This feeling of being incredibly full is actually something my brain wants, and it's really hard to say no. When you begin to diet, you almost never feel "full", just satiated, or "not hungry". It gets to the point sometimes where being full is all you can think about, all you want, and you can't take your mind off of it. Eating addiction is a real thing. With enough willpower, it's pretty easy to lose weight, BUT, if your life is taking a shit on you, getting that willpower can be difficult.