This one has a super simple answer. They eat too many carbs.

When carbs spike their blood sugar, insulin swoops in to convert the excess glucose into fat. When they try to restrict calories, they focus on cutting the wrong macros. Calories from carbs are hard to avoid and easily miscounted/underestimated unless you're literally weighing the food and know every ingredient used.

You're not even wrong that women are more likely to have a sweet tooth and fall for crap diets like "low fat" or "plant based". It's decades of evil marketing at play here. The result is not only that they stay fat, but then lose muscle and the number on the scale doesn't budge.

You know what's a whole lot easier to eyeball and get full on? Meat and veg you cooked yourself. That's the easiest solution by miles, but people will continue serving themselves way too much pasta and bread (because it looks right on the plate), adding sauces that are smuggling even more carbs, eating ultraprocessed junk loaded with sugar you can't even taste or filler carbs to cut production cost, and saying their occasional piece of "fun size" candy or favorite drink "doesn't count".