They don't
It doesn't
Roy Taylor has presented a very compelling idea that we each have a personal fat threshold, related to the amount of fat that can be stored naturally and in the natural places. What differs between people is where this threshold ends and where fat starts to be stored ectopically i.e where it shouldn't be e.g liver, where it contributes to insulin-resistance. Fo some it can take being hugely overweight before fat gets stored where it shoudn't be, but it seems to work the other way also. This explains why even those who are not visibly overweight can begin to store fat in the liver etc. They would just seem to have their threshold at a much lower level of excess fat...or so the theory goes.
This is a much deeper and very nuanced conversation to have. Perhaps in a another thread.
Perhaps it feels like that, and in some cases maybe it is. But we are all constantly learning about this and other health issues, and how much we should be expected to control vs how much we learn about why control, in many circumstances, is seemingly impossible.