the guidelines are not wrong.
people just have no idea how to actually lose weight. ask anyone off the street, their idea of a normal portion is completely off, they think a cheat meal is ok when they've 'eaten healthy' (read: ate a salad with more dressing than lettuce) and generally have no clue just how many calories they're consuming. exercise machines overestimate calories burned by a buttload, and calorie counts on nutrition labels are allowed a 20% margin of error. that adds up.
what's "changed" is portion size. people are eating too much, plain and simple. they're addicted to sugar and are too lazy to fix it.
it doesn't help that no one understands the fact that healthy =/= low calorie. Weight loss is simple, its just cico yall. All that keto, low fat, high protein, whatever, it doesn't affect anything. Macros don't matter at all when it comes to losing weight. Example: nuts. Super good for you, but a cup of walnuts (and most people eat more than a cup) is 523 calories. That's literally almost HALF of my maintenance calories, (but then again, im short + female so literally everyone has a higher maintenance than me). you can say you're on a diet because you're "eating healthy" but that doesn't mean anything. you can gain weight by eating healthy, and most people have no clue what healthy even is. exercise too, doesn't really mean anything for weight loss. sure, it helps, but the food is what really determines the outcome. you can eat a "healthy" bagel slathered in cream cheese and easily consume 500+ calories, but then you need to hike for ~2 hours to burn it off (and that's assumming you're a 135lb woman, if you weigh less it'll take even longer).
you dont get sustained weight loss through a diet. if you start out by eating french fries, lose ten pounds crash dieting, then go back to your old ways because you've lost the weight already, those ten pounds are gonna come back on. you lose weight and keep it that way by dieting for the rest of your life. of course, no one wants to accept that. they want quick and easy solutions or just blame everything on their metabolism yada yada. literally just eat less