Great question.
There are lots of reasons, and all of us know them. I think the main reason is a combination of (a) human nature - we are programmed to happily eat and enjoy our food, (b) a sick combination of addictive substances in our food, plus (c) "addiction" to said substances is not only socially-sanctioned, but denied by many to even be a problem. Addicts are blamed as being weak and having no discipline by those whose metabolism allows them to eat all they want, naturally limit themselves, and not gain weight. "Born at the finish line and they won the race," as the saying goes.
Think about it: if we all had to live off the land, grow our own produce, and raise our own livestock, very few people would be fat. IMO, it's the ubiquitous cheap and toxic carbage that tipped the scales over the last few generations in terms of the obesity/T2D epidemic. (Pardon the pun.)
This is why I've always said the most important part of committing to a weight loss plan is making all of the other supporting changes in your life. If you try white-knuckling it, you are very likely to fail. Most of us have done that countless times. Hey, if it didn't work the first 15,000 times, maybe you should change your strategy!
By "other supporting changes", I mean getting the people you live with on board - if not doing it with you, then at least supporting you. Change your habits at work. Change your social habits. Change your spending habits. Change how you spend your spare time - ie spend more time cooking and less time doing things that lead to mindless overeating. Learn to stand up to people and say no. NO is a complete sentence. Don't give anyone a reason or an hint that the "old" you is going to come back eventually, so they'll just be waiting for you to fail.
Some quick 6:30 am thoughts.