A lot of diets only focus on what you eat, and not your general lifestyle when your general lifestyle is the determining factor. If you eat healthy and exercise correctly, but only get 4-5 hours of sleep every night, you will not lose weight. And without the exercise part, you will probably gain weight even if you are eating "healthy". There's also the risk of sleeping too much also being linked to weight gain. Burning the midnight oil every now and then is ok, but making a habit of it definitely is not.

I used to stay up late coding all the time, because laying in bed about to go to sleep for some reason makes my brain think of solutions, and then I would only get about 4 hours of sleep pretty consistently. That scumbag brain meme comes to mind.

IIRC there are some other lifestyle choices that are linked to being overweight, like having a long commute. A long commute is probably putting yourself under a lot of stress hormones like cortisol for extended periods of time, and cortisol levels are linked to unhealthy weight gain.