This is simply not how obesity works. Yes initially it's the person's own fault but when it happens, it is self-reinforcing and sticky and can't really be countered by any (previously) known lifestyle or medical interventions short of yes, stomach surgery or a couple kinds of pills approved in 1980s that turned out to be too harmful to be used and were later retracted.
Nobody knows why some people are obese and some aren't. It may start with willpower and then reinforce metabolism, sure. Or maybe you caught a virus when you were two years old that disposed you to be fat for life. Or for whatever reason, a specific flora of gut bacteria grew in your belly. We just don't know yet.
I get that in very rare cases this is hard but it is possble.